Process for spectrophotometric analysis

ABSTRACT

A process is provided for obtaining spectral information and quantifying the physical properties of a sample. The process comprises launching polychromatic light having a wavelength ranging from about 100 nanometers to about 2500 nanometers alternately through at least one sample channel and at least one reference channel, through at least one high-efficiency fiber optic switch. The sample and the polychromatic light along the sample channel are directed to a sample cell wherein the polychromatic light is passed through the sample, producing sample spectral information. The polychromatic light directed along the reference channel produces reference spectral information. The sample and reference spectral information is reproducibly and uniformly imaged by passing or conveying said sample and reference spectral information through a mode scrambler and the uniformly imaged sample and reference spectral information is then processed in a wavelength discrimination device wherein the uniformly imaged spectral information is separated into component wavelengths and the light intensity at each wavelength determined and recorded. A chemometric model and the separated and recorded uniformly imaged sample and reference spectral information are then utilized to predict the physical properties of the sample. The process of the present invention provides improved prediction accuracy, increased reliability, lower operating costs, and is easier to use and calibrate.

BACKGROUND OF THE INVENTION

This invention relates to an improved process for obtaining spectralinformation and determining the physical properties of a sample. Moreparticularly, this invention relates to an improved process forutilizing spectrophotometry in an industrial environment, providingimproved prediction accuracy, increased reliability, lower operatingcosts, and easier use and calibration.

The physical properties of sample materials which, for purposes of thepresent invention encompass the physical, chemical, and fuel propertiesof sample materials, have historically been measured one property at atime, utilizing test methods which have been developed to specificallyevaluate one particular property. For example, the heat of formation ofa particular sample has been determined by actually burning the samplein a calorimeter. Similarly, the molecular weight of a sample has beendetermined by inducing and measuring viscous flow of the sample using aviscometer. In each of these examples, however, the physical testmethods measure, or quantify, the physical properties by actuallysubjecting the sample to the conditions in question. To measure morethan one physical property of a particular sample, a plurality of testsmust be individually conducted on a plurality of samples. Often thesesamples are destroyed or consumed in the process. These approaches tomeasuring physical properties are slow, expensive, subject to testinginconsistency, and do not facilitate on-line or real time use in anindustrial or field setting.

More recently, spectrophotometric analysis has been used to determineindirectly the quantitative properties of sample materials.

U.S. Pat. No. 4,800,279 to Hieftje et al. discloses a method forutilizing near-infrared absorbance spectra to identify the physicalproperties of gaseous, liquid, or solid samples. The method requiresmeasuring and recording the near-infrared absorbance spectra of arepresentative set of calibration samples and employing a row-reductionalgorithm to determine which wavelengths in the near-infrared spectrumare statistically correlated to the physical property being quantifiedand to calculated weighting constants which related the absorbance ateach wavelength to the physical property being monitored. Thenear-infrared absorbance of a sample can then be measured at each of thecorrelated wavelengths and multiplied by the corresponding weightingconstant. The physical property being quantified is then computed fromthe sum of the products of the absorbance of the sample and thecorresponding weighting constant at the correlated wavelengths.

Use of spectrophotometric analysis has numerous advantages over othermethods since it is rapid, relatively inexpensive, and multivariate inthat many wavelengths can be measured and therefore many properties canbe monitored simultaneously. While the potential for spectrophotometricanalysis in manufacturing facilities, chemical plants, petroleumrefineries, and the like is great, several obstacles must be overcome inorder to achieve successful implementation from a practical viewpoint.These obstacles include development of a process that is fieldresilient, accurate, and stable over time under generally adverseconditions.

Most spectrophotometers typically include a light source, a grating fordispersing light in a series of monochromatic, single wavelength beams,and a suitable photodetector. The grating may be positioned to providepredispersed monochromatic light to both the sample and then thedetector or, alternatively, polychromatic light from the source may bedirected onto the sample and then post-dispersed by the grating beforebeing directed to the detector. Post-dispersion permits analysis ofseveral wavelengths simultaneously.

Several U.S. Patents have illustrated the problems and progress madetowards development of the field rugged, accurate, and stablespectrophotometric devices and processes, each meting with varyingdegrees of success.

One method of improving the photometric precision of prior artspectrophotometers was to provide a stable light source for illuminatingthe sample under analysis. For example, U.S. Pat. No. 4,094,609 teachesa means for enhancing the consistency and uniformity of the light outputfrom the source used to irradiate the sample. However, even the bestmethods of providing a stable light source generally yield discerniblevariations in light intensity at the wavelengths of interest.

A subsequent improvement in photometric precision was made with theaddition of a reference spectral pattern which could be usedanalytically to account for variations in the light intensity of thesample spectral pattern which were not attributable to light interactionwith the sample. Spectrophotometers having a sample and referencechannel are referred to as dual-channel spectrophotometers. U.S. Pat.Nos. 4,820,045 to Boisde et al., 4,932,779 to Keane, and 4,755,054 toFerree teach use of fiber optic bundles, having multiple strands offiber optic cable with one or more stands dedicated exclusively forproviding a reference spectrum. It was subsequently found, however, thateach particular fiber in the fiber optic bundle, sampled a differentlocation on the filament of the light source and launched theirrespective transmitted light to different locations in the spectrometer.Small differences in the launching of light into and out of the fiberoptics created discernible artifacts in the measured sample andreference spectra. Minor variations caused by filament vibration, smalldifferences in intensity and color temperature along the length of thefilament, inhomogeneity in the optics, and other phenomena inducedifferent changes in the sample and reference spectra, which introducedsubstantial errors in quantifying the spectra. Moreover,spectrophotometric devices and processes utilizing fiber optic bundlesgenerally need to be recalibrated and the chemometric model rebuilt foreven the most trivial of maintenance tasks such as the routinereplacement of the light source. This model rebuilding step can requirethe collection and the analysis of from 20 to 100 representative samplesprior to proceeding with chemometric model rebuilding. These activitiesare costly and time consuming. Furthermore, fiber optic bundles are alsosubstantially more costly than single fiber optic strands.

Many of the inherent problems with fiber optic bundles were addressedwith the use of single strand fiber optic cable and means to launch thelight alternatively to and from the sample and reference channelsthrough the same fiber optic strand. The various means for alternativelydirecting light to the sample and reference channels are described inseveral U.S. Patents.

U.S. Pat. No. 4,938,555 to Savage teaches the use of a single fiberoptic strand having a moving mirror-type fiber optic light divertingmeans for directing light from a single fiber to one of a plurality ofselected locations. Moving mirror-type fiber optic light-divertingdevices generally present a number of obstacles to constructing andutilizing such a device, particularly in a manufacturing environment.The moving-mirror fiber optic diverting devices require that the lightlaunching fibers be precisely aligned with each receiving fiber. This isparticularly difficult and requires several critical multidimensionalalignments of the optical components. These critical alignments are alsoparticularly vulnerable to vibrationally-induced misalignment, a commonconcern in a field or manufacturing environment. These criticalalignments are also subject to wear in the mechanical devices used todrive the mirror and select among the multiple ports of the divertingmechanism.

Moreover, the efficiency of transmitting light power from the lightlaunching to receiving fibers is inherently low in such light-divertingdevices. As described in U.S. Pat. No. 4,820,045 to Boisde et al.,"apart from the transmission loss due to the actual fibre, there arecertain light energy loss causes at the junctions of the fibres,(particularly in collimating lenses and also during reflections at theintake of the fibres)."

Another fiber optic light-diverting means employs the use of a chopperor shutter to selectively launch light from a single fiber to aplurality of receiving ports. An example of such a light-diverting meansis disclosed in U.S. Pat. No. 4,755,054 to Ferree wherein a rotatablechopper means is used to direct light from a plurality of light sourcesto a receiving fiber. While the chopper and shutter devices reduce theneed for a critical moving optical component, chopper and shutterdevices transmit light power from the light launching to receivingfibers at a particularly low efficiency. The light efficiencies inherentto these devices can be, and are generally lower than 50% of the lightpower introduced into the launching fiber.

Another limitation attendant to many previously disclosedspectrophotometric devices and processes is in the means used to resolveand measure light at the various and particular wavelengths. Forexample, in International Application published under the PatentCooperation Treaty WO 90/07697 to Lefebre, wavelength resolution isachieved using a moving grating monochromator. The moving gratingmonochromator comprises a diffraction grating rotating about a centralaxis relative to the light source for alternately projecting light fromeach narrow wavelength band onto the fiber optics leading to the sampleand reference channels and subsequently to the detector. The precisionand reliability of the device is limited by the repeatability of themechanical motion of the diffraction grating. Wear on the mechanicalcomponents used to produce the motion, such as bearings, ultimatelylimits the wavelength precision of spectrophotometers of this design andeventually necessitates recalibration of the analyzer and development ofa new chemometric model.

Alternative monochromator means have been developed to correct some ofthe deficiencies associated with monochromators having a movingdiffraction grating. For example, some previous spectrophotometers haveemployed various types of filters rather than diffraction gratings forwavelength resolution. U.S. Pat. No. 4,883,963 to Kemeny et al., teachesthe use of an acousto-optical tunable filter (AOTF) for singlewavelength resolution. While an AOTF obviates the need for a criticalmechanical motion device in the monochromator, these devices share acommon limitation with moving diffraction grating monochromators in thateach wavelength in the spectrum must be analyzed sequentially and cannotbe analyzed simultaneously. This limits the speed with which thespectrum can be measured and increases the cycle time for providingdeterminations from the analyzer. Moreover, the sequential scanning ofthe wavelengths can introduce artifacts and irreproducibilities,particularly in a field environment when, for example, gas bubblesappear in the sample or the sample composition changes abruptly.

We have now found that many of the previously disclosed fiber opticbundle and light diverting devices and processes thereof introduceartifacts into the spectrum which degrade the accuracy and precision ofthe analysis. These artifacts can occur from the imprecise orirreproducible imaging of light from the launching into the receivingfiber optics and generally appear as irreproducibilities in the measuredspectrum. Relatively small misalignments in the imaging can result insignificant irreproducibilities in the measured spectrum, with theseirreproducibilities being particularly significant when thespectrophotometric process is used to determine the physical propertiesof samples. While these irreproducibilities may be less likely to occurin a controlled laboratory environment, they are assured in anindustrial facility where on-like analysis can place an analyzer underparticularly harsh conditions.

We have found that a process utilizing a mode scrambler, particularly inspectrophotometric processes using single fiber optic strands for thetransmission of light to and from the sample, mitigates many of theeffects caused by irreproducible imaging and small misalignments, whichsignificantly improves the precision of spectral measurements. Theseirreproducibilities and misalignments cause unpredictable andnon-uniform changes in the angular distribution of light launched fromthe fiber optic strand into the wavelength resolving device utilized inthe spectrophotometric processes. By scrambling or redistributing themodes of light projection in the fiber optic strand, the mode scramblingstep provides a uniform and reproducible image of the light from thefiber optic strand into the wavelength resolving device of thespectrophotometer, which is essential for obtaining precise spectralmeasurements.

We have also found that a process utilizing high efficiency fiber opticswitches in tandem with a mode scrambling device to reproducibly anduniformly image light into the spectrophotometer, provides a substantialimprovement in chemometric prediction precision. It has similarly beenfound that the a process comprising a spectrophotometer having a fixeddiffraction grating, a single fiber optic strand for launching lightfrom the sample and reference channels into the spectrophotometer, andan array of photodetectors for measuring the light intensity at multiplewavelength simultaneously, provides a substantial and furtherimprovement in chemometric prediction precision, speed of analysis, andprocess versatility.

It is therefore an object of the present invention to provide a processfor chemometric prediction with superior chemometric predictionaccuracy, reliability, resilience, and stability over time, suitable foruse in a manufacturing or field environment.

It is another object of the present invention to provide a process forchemometric prediction without the inherent cost and inaccuracies offiber optic bundles.

It is another object of the present invention to provide a process forchemometric prediction that efficiently and precisely measures the lighttransmitted through the sample and reference channels of the analyzerwith desensitized fiber optic switches and without other divertingdevices that rely on the precise mechanical alignment of a criticaloptical component.

It is another object of the present invention to provide a process forchemometric prediction that precisely, reproducibly, and expeditiouslymeasures absorbances at all relevant wavelengths and is not limited tosequential wavelength measurement.

Other objects appear herein.

SUMMARY OF THE INVENTION

The above objects can be obtained by providing a process for obtainingspectral information and quantifying the physical properties of a samplein accordance with the present invention. The process compriseslaunching polychromatic light having a wavelength ranging from about 100nanometers to about 2500 nanometers alternately through at least onesample channel and at least one reference channel, through at least onehigh-efficiency fiber optic switch. The sample and the polychromaticlight along the sample channel are directed to a sample cell wherein thepolychromatic light is passed through the sample, producing samplespectral information. The polychromatic light directed along thereference channel produces reference spectral information. The sampleand reference spectral information is reproducibly and uniformly imagedin a mode scrambler and the uniformly imaged sample and referencespectral information is then processed in a wavelength discriminationdevice wherein the uniformly imaged spectral information is separatedinto component wavelengths and the light intensity at each wavelengthdetermined and recorded. A chemometric model and the separated andrecorded uniformly imaged sample and reference spectral information arethen utilized to predict the physical properties of the sample.

The present invention provides a process for obtaining spectralinformation and quantifying the physical properties of a sample thatachieves superior chemometric prediction precision and is particularlyreliable, resilient, and stable over time. A process in accordance withthe principle of the present invention can generally achieve aprediction accuracy for measuring composition of better than plus orminus 1.0%, better than plus or minus 0.5%, better than plus or minus0.2%, and even consistently better than 0.1%. The process utilizessubstantially no moving optical components other than the highefficiency optical switches and is designed to factor out, substantiallyreduce, or mitigate imprecision in chemometric predictions generallyincurred by prior art devices and processes. When repairs are necessary,such as the routine replacement of the light source or repairs requiringmovement of the device or fiber optics, the process and device aredesigned to accommodate many of these repairs without requiringrecalibration of the instrument or redevelopment of the chemometricmodel.

The present invention provides a process for obtaining spectralinformation and quantifying the physical properties of a sample thatdoes not utilize fiber optic bundles or incur the inherent costs andimprecision of fiber optic bundles. By eliminating use of fiber opticbundles, discernable artifacts, caused by small differences in thelaunching of light into and out of each fiber strand in the fiber opticbundle are eliminated. Elimination of fiber optic bundles furtherreduces the errors in quantifying the physical properties from spectrawhich are induced by these artifacts and changes in these artifacts overtime, and reduces the need to recalibrate and redevelop the chemometricmodel upon routine maintenance of the analyzer. Moreover, fiber opticbundles can cost from 10 to 100 times more than a single fiber opticstrand.

The present invention provides a process for obtaining spectralinformation and quantifying the physical properties of a sample thatefficiently and precisely measures the light transmitted through thesample and reference channels of the analyzer with desensitizedhigh-efficiency fiber optic switches and without other diverting devicesthat rely on the precise mechanical alignment of a critical opticalcomponents. The process of the present invention utilizeshigh-efficiency fiber optic switches resulting in a higher signal tonoise ratio and improved photometric precision. The high-efficiencyfiber optic switches also substantially reduce the time required tomeasure the spectrum. The benefits of the high-efficiency fiber opticswitches are accommodated and complemented by a mode scrambler, whichcompensates for the irreproducibilities in the imaging of light acrossthe high-efficiency switches, which can be caused by misalignments inthe fibers of the high-efficiency fiber optic switches upon cycling.

The present invention provides a process for obtaining spectralinformation and quantifying the physical properties of a sample thatprecisely and expeditiously resolves the light intensity at all relevantwavelengths and is not limited to the measurement of light intensity ateach wavelength in sequence. The process of the present invention canmeasure the spectrum of a sample at any or all of the relevantwavelengths of interest simultaneously, which results in a substantialincrease in the speed of spectral analysis and eliminates artifactscaused by abrupt changes in the composition of the sample. Fasterspectral analysis results in faster sampling cycle rates and permits theanalysis of a plurality of samples using the same analyzer device.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagram of a spectrophotometric device in accordance withthe principles of the process of the present invention.

FIG. 2 is a diagram of a multiplexing embodiment of a spectrophotometricprocess in accordance with the principles of the present invention.

FIGS. 3A and 3B are diagrams of a sample cell embodiment of aspectrophotometric process in accordance with the principles of thepresent invention.

FIG. 4 is a diagram of a sample cell orientation wherein the sample isdirected to the sample cell through a slip-stream in accordance with theprinciples of the present invention.

BRIEF DESCRIPTION OF THE INVENTION

The present invention refers to an improved near infraredspectrophotometric process, particularly suited for determining thephysical properties of a sample in an industrial environment. For thepurpose of the present invention, spectral information refers to lighthaving wavelengths in the ultraviolet (100 nanometers to about 400nanometers), visible (400 nanometers to about 800 nanometers), andnear-infrared (800 nanometers to about 2,500 nanometers) regions.Spectral images, for purpose of the present invention, are theparticular spectra or segments of spectra, often described as therelationship of optical wavelength, frequency, or the like (x-axis) andabsorbance, transmittance, light intensity, or the like (y-axis),corresponding to a particular spectrophotometric analysis.

The optical features of the near-infrared range, a range particularlysuited for the analysis of the physical properties of hydrocarbons, aregenerally combinations and overtones of vibrational modes found in theinfrared region (2,500 nanometers to about 25,000 nanometers).Generally, asymmetric bonds having dipole moments create detectable anddistinguishable features in the near infrared region. In particular,combinations and overtones associated with the fundamental infraredabsorbance associated with the bonds H-X, where H is hydrogen and X iscarbon, nitrogen, oxygen, or sulfur, give particularly intense features.Three overtone bands of the H-C stretching mode and three combinationbands of C-H stretching and bending modes are found in the near infraredregion. Each set of overtone and combination bands contain similarinformation.

Since some bands in the near-infrared range contain similar information,a narrower frequency range can be utilized to obtain accuratedeterminations of physical properties. Generally, any overtone band,combination band or combination of overtone and combination bands can beutilized, however, a particular range is generally preferred dependingon the system under analysis. For example, for the analysis oftransparent petroleum liquid products, the wavelength range of betweenabout 850 nanometers to about 1000 nanometers and spanning the thirdcarbon-hydrogen stretching overtone is particularly useful because theoptical path length preferred for analyzing these samples is at theconvenient value of about 10-200 millimeters. Crude petroleum streamscan be preferably analyzed over wavelength ranges spanning thecarbon-hydrogen combination band of from about 1300 nanometers to about1500 nanometers and the first carbon-hydrogen stretching overtone ofbetween about 1600 nanometers to about 1800 nanometers. These higherwavelength ranges can be preferred because crude petroleum oils, unlikerefined petroleum products, have several strong absorbances due toelectronic transitions occurring below 1200 nanometers which interferewith and obscure the combination and overtone bands rendering thespectra below 1200 nanometers less useful for chemometric prediction.

Traditionally, spectrophotometric analysis has been used to determinethe qualitative nature of compounds and complex mixtures from theirspectral information. However, physical properties can also bequantitatively correlated to spectral information where the property isrelated to the composition or heuristically correlated to the spectra,even when the property is not obviously related to composition.Quantitative determinations are also well suited to spectrophotometricanalysis and in particular, spectrophotometric analysis utilizingnear-infrared. According to the Beer-Lambert law, absorbance (A) isproportional to the weight fraction of the absorbing species (W) asdescribed in the following equations:

    A=Log (l.sub.o /l)=εLρW

where l_(o) is the light intensity incident on the sample, l is thelight intensity transmitted through the sample, ε is the absorptioncoefficient, L is the path length through the sample, and ρ is the bulkdensity of the sample. The transmittance is defined as the ratio ofl/l_(o). The absorbance can also change, indirectly, with thetemperature of the sample due to thermal expansion, dissociation of thehydrogen bonds, and changes in the populations of energy levels whichare associated with the absorption of light intensity. Analogousexpressions to the above can be developed for reflectance spectra sothat the properties of opaque substances can be similarly correlatedwith spectral information.

The spectral correlations developed for use in spectrophotometricprocess in accordance with the present invention are generally builtutilizing most or much of the spectrum of the sample although suitablecorrelations can also be developed using the absorbances measured at afew select wavelengths. Although a spectrum can consist of severalhundred intensities measured at different wavelengths, many of thesedata points are highly interdependent, or colinear. Multivariateregression can be used to simplify the spectrum into latent variableswhich describe the independent variations in the spectra for a set ofsamples. The scores or relative magnitudes of the latent variables inthe spectrum change as the properties of the sample change. The numberof latent variables necessary to accurately model a system generallydepends on the system being analyzed. Generally, the properties can bemodeled using less than 20 latent variables, frequently less than 10latent variables, and often less than 6 latent variables. The number oflatent variables minimally necessary to predict stream properties can beestimated using splitting techniques, PRESS statistics, by plots ofvariance fit using successive numbers of latent variables, or otherforms of statistical analysis.

A spectrophotometric process in accordance with the principals of thepresent invention, can span a broad wavelength range of from about 100nanometers to about 2,500 nanometers where the particular feedstocknecessitates absorption determinations at a wide range of wavelengths.Generally, the spectrophotometric process spans a range of from about100 nanometers to about 2,500 nanometers, as narrow as from about 800nanometers to about 1,100 nanometers, or even as narrow as from about850 nanometers to about 1,000 nanometers.

Spectrophotometric process in accordance with the present invention andspanning the range of from about 850 nanometers to about 1,000nanometers include the third overtone of the carbon-hydrogen stretchingmode (850 nanometers to about 960 nanometers) and the second overtone ofthe oxygen-hydrogen stretching mode (960 nanometers to about 990nanometers). The third combination band of the carbon-hydrogenstretching and bending modes is found at 1000 nanometers to about 1120nanometers. However, the spectra of the third combination band can benoisy when the photo-response is measured on silicon detectors, found inphotodiode arrays attendant to many spectrophotometric processes.Spectrophotometric processes measuring the properties of hydrocarbonsand oxygenated hydrocarbons that are found in transparent liquidhydrocarbon products generally analyze the properties of such a sampleacross a wavelength range of about 800 nanometers to about 1100nanometers. Similarly, the more useful spectral ranges for analyzingcrude petroleums and other black oils are from about 1300 nanometers toabout 1500 nanometers and/or from about 1600 nanometers to about 1800nanometers, depending on the extent to which absorbances due toelectronic transitions interfere with these vibrational bands of thehydrocarbon oil and the property being monitored.

FIG. 1 is a diagram of a spectrophotometer in accordance with theprincipals of the process of the present invention. The light source 1can be any suitable polychromatic light source such as mercury vapor, ordeuterium lamps, or tungsten filament lamps filled with an inert gassuch as, but not limited to, krypton, with the filament under vacuum orwith a halogen at low pressure. The preferred polychromatic light sourceis a tungsten-halogen lamp powered by a constant voltage or constantcurrent supply. The output of light source 1 is launched through singlelens 2 into fiber optic cable 3.

Fiber optic cable suitable for use in the spectrophotometric process ofthe present invention has a core diameter ranging from about 50 μm toabout 1000 μm, preferably from about 100 μm to about 400 μm, and morepreferably from about 100 μm to about 250 μm, for best results. Thepreferred material for the core of the fiber optic cable is silica witha low hydroxyl content of below about 50 ppm, or any other material thatis substantially transparent to light in the wavelength region ofinterest. The fiber optic cable can be externally cladded for lightreflection to maintain the light within the fiber optic cable. Suitablecladding materials for silica fibers can include doped glasses,fluorocarbons, and mixtures thereof. The preferred fiber optic cablecladding materials are fluorocarbons for conducting light through thehigh-efficiency fiber optic switches 4 and mode scrambler 15, whereasthe preferred cladding material for long fiber optic runs, such as fromthe light source 1 to the upstream high-efficiency fiber optic switch 5and from the downstream fiber optic switch 6 to the mode scrambler 15,is doped glass. Single fiber optic stands are preferred over otheralternatives, including fiber optic bundles, in the process of thepresent invention. Fiber optic strands generally reduce the cost of thespectrophotometric process, are more resilient, improve flexibility ofapplication, and provide for more precise photometric measurement.

The routing of light through the optics is controlled by at least oneand preferably two high-efficiency light switches 4 consisting ofupstream light switch 5 and downstream light switch 6. Alternatively, asingle high-efficiency switch 5 and an optical coupler 6, or vice versacan be used. An optical coupler is a device that directs light from twoor more fiber optic cables into a single fiber optical cable or viceversa. Common coupling devices include, but are not limited to fusionsplices, mechanical splices, and passive taps. While coupling devicescan be used, orientations featuring optical couplers are generally lessefficient and are not preferred. In the process of the presentinvention, fiber optic switches and/or couplers are used in place ofshutters, choppers, beam splitters, moving mirrors, and other meanscommon to prior art spectrophotometers. The fiber optic componentsprovide the advantages of low cost, reliability, ruggedness, and reducedsize, particularly beneficial for process analyzer applications commonin a field or manufacturing plant environment.

The polychromatic light from light source 1 can be directed in either oftwo primary directions: the sample channel direction or the referencechannel direction. The spectrum of light intensity is alternativelymeasured through the sample and reference channels in order tocompensate for photometric drift. Polychromatic light generally travelsthrough the sample channel direction when the high-efficiency lightswitches 4 are aligned to direct the polychromatic light to the samplecell 7 through fiber optic cable 8 and return the polychromatic light todownstream light switch 6 through fiber optic cable 9. Polychromaticlight passes through the sample cell 7 where it is passed through samplestream 23 to be analyzed, strikes a prism or retroreflector 10, passesthrough sample stream 23 again, and is returned to fiber optic cable 9.Alternatively, the light can be directed through the sample in a singlepass cell where the light is imaged from fiber optic cable 8, throughthe sample, and into fiber optic cable 9 using at least one single ormulti-element lens. In the preferred embodiment, two lenses are used tocouple the light between fiber optic cables 8 and 9, and to collimatethe light passing through the sample.

The spectrophotometric process of the present invention can have aplurality of sample channels and reference channels. Thespectrophotometric process can also have a plurality of sample channelswith one reference channel. The channels can gain access to thepolychromatic light source through a fiber optic manifolding system. Inthis manner, the same primary spectrophotometric hardware can beutilized to analyze a plurality of sample streams for a plurality ofphysical properties. This system can be controlled by multiplexingdevices that function utilizing switching devices including, but notlimited to, mechanical motion mechanisms that align the fiber opticslaunching and receiving light.

FIG. 2 is a diagram of such a multiplexing device. The polychromaticlight from a light source is directed through conduit 201 where it canbe switched or multiplexed to any of conduits 202, 203, 204, 205, or206. Receiving conduits 202 through 206 can be conduits for directingthe polychromatic light to either one or more sample or referencechannels.

The high-efficiency fiber optic switches suitable for use in thespectrophotometric process of the present invention are generallymechanical motion mechanism types and solid state types where a liquidcrystal becomes either transparent or reflective through the applicationof an electric or electromagnetic field. The preferred high-efficiencyfiber optic switches are latching type mechanical motion switches. Thelatching type switches generally work by moving a fiber optic cable fromport to port and rely on the alignment of the launching and receivingfiber optics to maintain efficient transmission of the light. Generally,no optical elements are placed between the launching and receiving fiberoptic cables. However, optical elements, and in particular, one or moresingle or multi-element lenses, could be placed between the launchingand receiving fiber optic cables, but this can compromise the efficientcoupling of light between the fiber optic cables. The high-efficiencyfiber optic switches can cycle between sample and reference ports atfrequencies ranging from about 100 to 0.001 cycles per second,preferably from about 10 to about 0.01 cycles per second, and morepreferably from about 1 to about 0.1 cycles per second, for bestresults. Insufficient sample and reference cycling may not provideadequate reduction of photometric drift while excessive sample andreference cycling can introduce additional noise in the spectrum. Thehigh-efficiency fiber optic switches of the present invention couplemore than 50%, generally more than 75%, and often more than 90% of thelight form the launching fiber into the receiving fiber.

The sample cell, suitable for use in the spectrophotometric process ofthe present invention can be designed for insertion into the samplesource directly or can be accessed by conveying a slip-stream of thesample from the sample source to the sample cell utilizing a conveyingdevice or conduit such as sample tubing. Suitable locations forinsertion of the sample cell, can be, but are not limited to pipelines,process vessels, distillation towers, and the like. The preferablelocation for sample cell insertion is generally at a reasonably constantstream temperature and pressure, with fluid turbulence in the opticalpath of the sample cell kept to a minimum. These parameters are notessential to obtaining accurate analyses but can improve the quality andconsistency of the results.

The sample cell itself, can be designed to direct light through thesample in a manner such that light passes through the sample at leastonce and preferably twice. Where sampling is done through use of aslip-stream, the sample cell can be a once through single pass typewhere the fiber optics that launch and receive light are located atopposite ends of the cell and one or more lenses are used to directlight in and out of the two fiber optic cables. Where sampling is donethrough use of fan insertion probe, two or more passes are generallypreferred such that the fiber optic cables launching light into, andreceiving light from the sample cell can be placed at the same end ofthe sample cell. The polychromatic light can be reflected at the end ofthe sample cell opposite the fiber optic cables, in a two pass samplecell, where the reflecting prism reflects the light at least twice andpreferably three times in order to insure accurate reimaging on theoutlet fiber optic cable. The sample can be isolated from the fiberoptic cables by a plurality of windows and preferably two windows sealedon O-rings or gaskets. The total path length of the sample, describedpreviously as L, is generally between about 0.1 mm and 1000 mm,preferably between about 10 mm and 300 mm, and more preferably betweenabout 50 mm and 200 mm, for best results. Shorter path lengths canresult in inadequate light absorption by the sample, while excessivelylong path lengths can result in over-absorption of light and impreciseresults.

FIGS. 3A and 3B are views of a sample cell having a reflecting prism forreflecting light three times in accordance with the present invention.The sample to be analyzed is passed through sample cell 300 throughconduit 306 and exits the sample cell 300 through sample exit conduit307. Light beam 301 enters the sample cell 300, passes through thesample entering the sample cell 300 through conduit 306, and isreflected three times by walls 302, 303, and 304 of reflecting prism308. The light beam 305, reflected by prism wall 304, is directed backthrough the sample and exits the sample cell 300.

FIG. 4 is a diagram of an insertion probe-type and a slip stream-typesample cell in accordance with the present invention. The sample streamfrom conduit 400 is conveyed to slip stream-type sample cell 402 throughsupply conduit 401, located externally to the sample conduit 400. Thesample is returned to sample conduit 400 from sample cell 402 throughreturn conduit 403. The sample stream 400 is conveyed to insertionprobe-type sample probe 404 directly without the need for externalconduits since the sample probe 404 is inserted directly into the sampleconduit 400.

The polychromatic light from light source 1 can also travel through thereference channel direction when the high-efficiency light switches 4are aligned to direct the polychromatic light to the attenuator 11through fiber optic cable 12 and return the polychromatic light todownstream light switch 6 through fiber optic cable 13. The attenuatoris utilized to balance the polychromatic light transmission between thereference and sample channels and to ensure reproducible imaging of thelight into the fiber optic cable 13.

Attenuation devices suitable for use in the spectrophotometric processof the present invention can utilize one or more lenses, fiber opticcable, and can be utilized without optics by separating the two fiberoptics with one or more coupling devices. The preferred attenuationprocess utilizes of two plano convex lenses or two achromatic doubletlenses in series, with screens or other devices optionally placedbetween the lenses to provide achromatic attenuation of light launchedbetween the fiber optic cables.

The polychromatic light passing through downstream high-efficiency lightswitch 6 is routed through fiber optic cable 14 to a mode scrambler 15.High-efficiency fiber optic switches, upon cycling, may not providereproducible imaging of the light into the spectrograph. This can resultfrom mode noise whereby the angular distribution of light emerging fromthe fiber optic into the spectrograph is sensitive to slightmisalignment of the launching and receiving fibers in thehigh-efficiency fiber optic switches. This problem is solved byproviding a mode scrambler 15 to reproducibly and uniformly image lightfrom the fiber optic cable into the spectrograph 17.

The mode noise occurs because the fiber optics are optical elements withimaging properties and are not merely conduits for transmitting lightthrough the spectrophotometer. When the mechanical alignment of thefibers in the high-efficiency fiber optic switches, or the alignment ofany other optical elements between the light source and spectrograph ischanged, the image launched from the fiber into the spectrograph can bealtered. This irreproducibility is manifested as noise in the spectralimage measured on the photodetector. When light is passed throughseveral hundred feet of fiber optic cable, the image presented to thefiber is altered due to scattering by bends, cracks, and otherimperfections in the fiber. Thus, the image presented from a long run offiber optic cable is scrambled, and the light traveling through thefiber assumes an equilibrium distribution of modes or states in thefiber just as a fluid moving through a pipe develops an equilibriumvelocity distribution across the diameter of the pipe. Once theequilibrium mode distribution is obtained, the image emerging from thefiber optic cable is uniform in that the relative light intensityemerging at any angle relative to the axis of the fiber is substantiallyindependent of the way in which light was launched into the fiber and ischaracteristic of the type of fiber optic cable used to transmit thelight.

While several hundred feet of fiber optic cable may be used to affectmode scrambling, short lengths of fiber optic cable may be used ifbends, cracks, and other imperfections are systematically introducedacross the length of the fiber optic cable. For example, the modescrambler can comprise from about 1 to about 20 runs of fiber opticcable turned around two spools in a figure eight pattern, where thespools range in diameter from about 0.125 inches (12.7 centimeters) toabout 5.0 inches. The mode scrambling objectives can also be achieved bythe more costly means of providing a fiber optic cable run between thedownstream high-efficiency fiber optic switch 6 and the spectrograph 17of from about 10 feet (3 meters) to about 10,000 feet (3.023 meters),preferably from about 100 feet (3.0 meters) to about 5,000 feet (1511meters), and more preferably from about 200 feet (60 meters) to about2,000 feet (605 meters), for best results. It is understood that othermode scrambling means can be used to alter the image, or angulardistribution of light in the fiber optics, so that a uniform andreproducible image is obtained when the light is launched from the fiberoptic into the spectrograph; and the aforementioned mode scramblingmeans are intended to be illustrative rather than limiting.

In addition to compensating for slight irreproducibilities in themechanical alignment of the fibers, which occurs upon cycling thehigh-efficiency fiber optic switches, the mode scrambler also mitigatesthe effects of selectively altering the transmission of light througheither the sample or reference channels. Such effects are normallymanifested in the background spectrum, described below, and themagnitude of these effects is substantially reduced by the use of a modescrambler in the fiber optic network. Effects that can be mitigated bythe mode scrambler include those caused by bending the fiber opticcable, breaking and remaking the fiber optic connections, and refocusingthe optics in the sample cell or attenuator. Thus, the mode scrambler isgenerally useful in any spectrophotometer using fiber optics for thetransmission of light, where the reproducible imaging of light into orout of the fibers is important to obtaining reproducible spectralmeasurements.

The polychromatic light from the mode scrambler 15 is passed throughfiber optic cable 16 to the spectrograph 17. Although any wavelengthdiscrimination device or means for separating the polychromatic light bywavelength and detecting the light intensity at each wavelength can beused, the preferred means includes a fixed direction grating 18 and aphotodiode array detector 19. The polychromatic light from fiber opticcable 16 is launched onto the fixed diffraction grating 18 whichdiffracts and reflects the light onto the photodiode array detector 19.Spectrophotometers having additional optical elements in thespectrograph can also be used.

Photosensitive array detectors suitable for use with the process of thepresent invention generally measure light intensity at all points in thespectrum simultaneously. Each photosensor, or pixel, in the arraygenerally measures the light intensity over a narrow range of wavelengthso that, taken together, the signals from all photosensors in the arrayconstitute the spectrum. Pixels generally correlate to the preciselocations where light was diffracted on and reflected from, the fixeddiffraction grating. Photosensitive array detectors suitable for usewith the process of the present invention generally have at least 100pixels spanning the corresponding wavelength range of particularinterest, preferably at least 200 pixels, and more preferably at least500 pixels for best results. Corresponding spatial resolutions suitablefor use with the process of the present invention should be at least 1pixel per nanometer, preferably at least 2 pixels per nanometer, andmore preferably at least 4 pixels per nanometer for best results. Theoptical wavelength resolution of the spectrophotometer should be betterthan 8 nanometers, preferably better than 4 nanometers, and morepreferably better than 2 nanometers for best results. Suitablephotosensitive array detectors for use in the spectrograph in accordancewith the process of the present invention can comprise metallurgy suchas indium/gallium/arsenide, germanium, silicon, platinum silicide, withsilicon being the preferred metallurgy for transparent liquid petroleumproducts and indium/gallium/arsenide being the preferred metallurgy forcrude petroleums.

The photosensitive array in accordance with the spectrophotometricprocess of the present invention generally measures light intensity,which can be converted proportionally to an electrical signal. Suitablemeans for this conversion can include any one of a number of solid stateelectronic devices, including, but not limited to photodiode arrays,charge-coupled devices (CCD arrays), and optical sensor arrays, with thephotodiode array being the preferred means for converting light intoelectric signals. The solid state electronic device serves to convertthe light signal to an electrical signal which is proportional to thelight intensity. Thus the light intensity is converted to a electricalcurrent, voltage, or charge signal which can be used as a convenientmeasure of light intensity. In this manner, spectra are generallydisplayed with wavelength or pixel orientation on the x-axis and lightintensity as measured by electrical current, voltage, or change on they-axis.

Light intensity spectra are the raw spectral images generated in thespectrophotometer. The light intensity spectra are generally used whenspectral information and particularly chemometric models are to bepassed among spectrophotometers, because the mathematics for doing soare relatively straightforward. Transmittance and absorbance spectra arenon-linear combinations of the intensity spectra of the sample and thereference channels. These combinations, and in particular, absorbance,are most useful for generating chemometric models because absorbance isproportional to the concentration of the light absorbing material andthese combinations are less sensitive to drift and other aberrationscaused by imperfections in the spectrophotometer.

A spectrograph in accordance with the spectrophotometric process of thepresent invention, can utilize a temperature expansion resistantmaterials for mounting the optical elements, as compared to aluminummounting plates, which have previously been used in the art. Thecoefficient of thermal expansion of aluminum is about 23×10⁻⁶ /°C. andcan produce a spectral shift of as much as 1 nanometer or more for a 3"spectrograph chassis over a 60° C. temperature change. Use oftemperature expansion resistant materials can substantially minimizespectral shifts and increase resultant prediction accuracy. Thetemperature expansion resistant material suitable for use in the presentspectrophotometric process generally has a coefficient of thermalexpansion of less than 10×10⁻⁶ /°C., preferably less than 5×10⁻⁶ /°C.,and more preferably less than 2×10⁻⁶ /°C., for best results. Suitablematerials for use as the temperature expansion resistant mounting platecan include, but are not limited to carbon and graphite fibers in asuitable binder or matrix system. Examples of fiber matrix or binderssuitable for use in the process of the present invention can includeepoxy, thermosets, and thermoplastics. The preferred temperatureexpansion resistant materials are the carbon fibers and graphite fibershaving epoxy binders. A suitable material for use in the process of thepresent invention is Quasi-Isotropic Layup with P-75 Pitch-basedGraphite Fiber manufactured by Amoco Performance Products.

Driver and Interface boards 20 are provided to drive the photodiodearray detector 19, sequence the pixel readings, and amplify the signalsprior to the analog to digital converter (ADC) 21. The interface boards20 can comprise an amplifier section for conditioning the analog signalto the ADC 21, a crystal clock section for precisely sequencing thereading of the pixels, and/or a digital section for synchronizing thereading of the photo array detector and triggering the ADC 21. The scanrate for scanning the photodiode array director 19 can be adjustedthrough the driver and interface boards 20 and generally ranges fromabout 10 to about 10 million pixels per second, preferably from about1,000 to about 100,000 pixels per second, and more preferably from about10,000 to about 100,000 pixels per second for best results.

The ADC 21 converts the light intensities from the photodiode arraydetector 19 into digital signals and is designed to be compatible withcomputer 22 or other means to digitally process spectral information.The computer 22 is provided to acquire, store, display, and analyze thespectra.

A computer suitable for use in the spectrophotometric process of thepresent invention generally has a means for interfacing with the ADC,optional means for reading other analog signals, such as sampletemperature, digital I/O ports for controlling and sensing the status ofthe photodiode array and fiber optic switches, a central processing unit(CPU), random access memory (RAM), hard or floppy discs or other meansfor mass storage, a keyboard or other means for entering property datafor calibration, an optional display means for reporting and trendingdeterminations from the analyzer, and a serial port or other means fortransmitting determinations from the analyzer to a human or processcontrol interface.

Spectrophotometers similar to that described in FIG. 1 and in accordancewith the spectrophotometric process of the present invention generallyfunction by launching polychromatic light having a wavelength rangingbetween from about 100 nanometers to about 2500 nanometers, preferablyfrom about 800 nanometers to about 2500 nanometers, and more preferablyfrom about 800 nanometers to about 1100 nanometers, alternativelythrough at least one sample channel and at least one reference channel,utilizing at least one high-efficiency fiber optic switch 4. Thereference and sample channel cycling is performed in order toself-reference the process and substantially correct for photometricdrift.

The sample to be analyzed is directed to a sample cell 7 along withpolychromatic light from along the sample channel, whereby thepolychromatic light is passed through the sample. The polychromaticlight can be passed through the sample once or preferably at least twiceutilizing a prism or light reflection device. In multiply passing thepolychromatic light through the sample, the light can be reflected twoor more times. Some of the light is absorbed by the sample an some ofthe light passes or is transmitted through the sample and becomes samplespectral information.

The polychromatic light along the reference channel is attenuated inorder to balance the polychromatic light transmission between thereference and sample channels. The light that is transmitted through theattenuator 11 is termed attenuated reference spectral information.

The sample spectral information from the sample channel and theattenuated reference spectral information are routed alternately to amode scrambler 15 for reproducibly imaging light into the spectrograph17 in a subsequent step. The sample spectral information and theattenuated reference spectral information from the mode scrambler 15 isthen processed in a spectrograph 17 wherein the sample spectralinformation and the attenuated reference spectral information areseparated by wavelength using a wavelength discrimination means and thelight intensity for each wavelength is measured using a photosensitivedetector. In the preferred embodiment, a fixed diffraction grating 18and photodiode array detector 19 are used for wavelength discriminationand detection. The absorbance spectrum is then calculated from thediffracted and recorded sample spectral information and attenuatedreference spectral information, and an appropriate chemometric modelapplied to the absorbance spectrum to predict the physical properties ofthe sample.

The reference channel measurement steps are provided to permitcorrection of the device for physical or mechanical changes that occurupstream of the upstream high-efficiency light switch 5 or downstream ofthe downstream high-efficiency light switch 6. Physical changes in thedevice itself, the testing conditions, or any of a number of otherevents can create imprecision in the spectrophotometric readings. Thespectrophotometric readings are also subject to photometric drift overtime. Photometric drift can originate in the light source, the fiberoptic cables, or other optical components, as well as the electronicsutilized for measuring light intensity. Since polychromatic lighttravels through the same path through both the sample and referencechannels, but for the fiber optic cable and other devices between thehigh-efficiency fiber optic switches, physical and mechanical changesand photometric drift that occur upstream or downstream of thehigh-efficiency fiber optic switches 4 can be factored out.

The light intensity signal measured on the sample channel is generallyquantified by the following expression:

    S.sub.s =S.sub.d +a.sub.c a.sub.s g l.sub.o (e.sup.-A)

where e is the base of the natural logarithm, S_(d) is the dark signal;a_(c) is the attenuation, or reduction in the amplitude of the lightsignal in the optics common to the signal and reference channels; a_(s)is the attenuation in the optics unique to the sample channel; and a isthe photometric gain (i.e. light intensity to electrical signalconversion factor). The dark signal, S_(d), can be determined bymeasuring light intensity with the high-efficiency fiber optic switchesjuxtaposed with one of the switches directed towards the sample channeland the other switch directed towards the reference channel. The darksignal is measured and defined as the zero point of the light intensityor y-axis.

The light intensity signal measured on the reference channel isgenerally quantified by the following expression;

    S.sub.r =S.sub.d +a.sub.c a.sub.r g l.sub.o

where a_(r) is the attenuation in the optics unique to the referencechannel.

The apparent absorbance measured on a dual channel spectrophotometer isdefined by the following expression:

    A.sub.a =log[(S.sub.r -S.sub.d)/(S.sub.s -S.sub.d)]=A.sub.b +εLρW

where the background spectrum is defined as A_(b) =log(a_(r) /a_(s)) andreflects the difference in light absorbance between the reference andsample paths with no sample present. The apparent absorbance A_(a), thebackground spectrum, A_(b), the absorption coefficient, ε, and to someextent, the path length L, are wavelength dependent. The apparentabsorbance, A_(a), in the dual channel instrument in accordance with theprocess of the present invention is substantially independent of thelight intensity incident on the sample, l_(o), common mode attenuation,a_(c), and the photometric gain, g. In this manner, thespectrophotometric process of the present invention eliminates potentialerror sources by rendering them substantially irrelevant.

Filtering steps can be provided for removing high frequency noise andbaseline drift from the absorbance spectrum. The absorbance spectrum, inits unfiltered form, can be subject to high frequency noise, or thescattering of points above and below the absorbance spectrum generallycaused by the electronic or optics. The absorbance spectrum, in itsunfiltered form, can also experience baseline drift. Baseline driftoccurs where the absorption baseline or slope of the baseline begins todrift upwards or downwards. Baseline drift generally is a form of lowfrequency noise and can be caused by drift in the electronics,scattering by the sample, fouling of optical surfaces or thermalexpansion in the optics. High frequency noise and baseline drift can beeliminated by a filtering step.

Filtering high frequency noise can be performed by smoothing orweight-averaging adjacent data points. Common algorithms for smoothingspectra can include Fourier filters, best-fit polynomials, Gaussian orbinomial filters, and binning or adding groups or adjacent data points.Binomial filters are used most commonly since most of the methodsprovide similar accuracy while the binomial filters are easier toimplement. Suitable binomial filters can include utilizing from 1 toabout 20 smoothing iterations with a 1:2:1 weighting function, with 4 to8 smoothing iterations being preferred. The extent of high frequencyfiltering is generally controlled by varying the number of points in thesmoothing algorithm. Binning methods can also be attractive since fewerdata points can be used to represent the spectrum, thereby increasingcomputational speed.

Filtering low frequency noise or baseline drift is most commonlyperformed by differentiation. The extent of low frequency filtering isgenerally controlled by taking successive derivatives with onederivative being preferred for the present invention.

Chemometric models which relate the spectral data to the desiredphysical properties of the sample can be generated by any of the severalcurrently available methods or combinations thereof. The simplest ismultiple linear regression (MLR), where the absorbance, or a suitablemeasure of light intensity, at various wavelengths can be correlatedwith the physical property to be determined. Once the correlation hasbeen established, physical properties of subsequent samples can bemeasured based on correlation coefficients and the spectrum of thesample. MLR utilizes only a small portion of the information availablein the spectrum and such models can be more sensitive to errors causedby colinearity or intercorrelations of the absorbance determinationsmade at the various wavelengths of interest.

The methods of factor analysis and latent variable determination havebeen developed to use more of the information available in the spectrumand to manage the effects of colinearity. In principle componentregression (PCR), successive factors or latent variables are calculatedwhere successive latent variables are correlated to the spectra of thesamples in order of contribution to the total absorbance untilsubstantially all of the absorbance can be described using linearcombinations of the latent variable. Each of the latent variablescalculated in PCR is independent or orthogonal to the other latentvariables and therefore accommodates colinearity in the spectralinformation. Once the latent variables are determined, they are thencorrelated with the physical property of interest. In PCR, the spectralinformation is reduced to latent variables without consideration as tohow the latent variables relate to the property of interest.

In partial least squares (PCS), the latent variables are determined inorder of contribution to the total absorbance and one or more of thephysical properties that are the dependent variables. Thus, the latentvariables of a PLS regression are maximally correlated to both thespectral and physical property data. Once a chemometric model is builtusing MLR, PCR, PLS, or similar means, the model can be applied to thespectrum of an unknown to predict one or more of its physicalproperties. For example, a suitable software package that implementsthese methods and builds chemometric models is SpectraCalc by Galactic.The derived chemometric model is then tested using samples of unknownsthat were not used for developing the model.

The spectrophotometric process of the present invention functions bestwhen key parameters that can affect the prediction precision andaccuracy of the spectrophotometer are identified and controlled. Theseparameters to be controlled can include wavelength precision,photometric precision, photometric linearity, sampling time, changes inor adjustments to the optical path, high-efficiency fiber optic switchrepeatability, sample temperature, sample flowrate, and environmentaleffects.

Shifts in the wavelength axis or wavelength drift can occur as a resultof mechanical vibration or shock and changes in the temperature of thespectrograph. Wavelength drift compromises prediction accuracy bychanging the wavelength range corresponding to each detector element ofthe photo-sensitive array. The accuracy of chemometric predictions canbe noticeably effected where the spectrum is shifted by 0.03 nanometersor more. Where the spectrum is shifted by more than 0.3 nanometers, theresulting predictions may not be sufficiently accurate for use in someprocess services.

Methods to reduce or compensate for the effects of wavelength driftgenerally include improving spectrograph design, controlling orcompensating for changes in the temperature of the spectrograph, andisolating the spectrograph from or mitigating the effects of mechanicalvibrations or shock. A light source that emits over a narrow wavelengthrange, such as a laser diode can be integrated into the optics in orderto provide a reference point for compensating for wavelength drift.Methods may also be utilized to maintain a substantially constantspectrograph temperature. These methods can include an ambienttemperature controlled environment for the spectrograph and/or atemperature control system that monitors the spectrograph temperatureand either controls or compensates for temperature changesmathematically within the chemometric model. Where a system is designedto maintain a substantially constant spectrograph temperature, thespectrograph temperature should be maintained within a total range ofabout 20° F., preferably within a range of about 10° F. (5.6° C.), andmore preferably within a range of about 5° F. (2.8° C.) for bestresults.

Changes in photometric output can occur as a result of movement orchanges in the optics, particularly those in the sample and referencepaths between the high-efficiency fiber optic switches, change sin thelevel of stray light in the spectrograph (often caused by scatteringfrom the diffraction grating), noise, and non-linearity in thephoto-electronics. Changes in photometric output can compromisechemometric prediction precision by selectively emphasizing ordeemphasizing portions of the spectrum, which can alter predictions fromthe chemometric models. The degree of photometric precision required toprovide chemometric prediction precision can vary depending on theparticular drift or inaccuracy introduced. For example, stray lightlevels above 0.1% of full scale intensity, wherein full scale intensityis defined as the amount of light needed to saturate the photodiodearray detector during the period between readings of the detector, caneffect the precision of chemometric predictions, while noise levelsabove 0.03% of full scale intensity can effect chemometric predictionprecision. Therefore, the degree of photometric precision required forprecise chemometric prediction generally depends on the particularsource of the photometric drift. Generally, photometric precision levelsof less than 0.03% of full scale intensity can ensure precise andreliable chemometric prediction performance in the spectrophotometricprocess of the present invention.

Methods to ensure photometric precision generally constrain the designof the spectrophotometer. These methods include the design on thephotoelectronics to maintain photometric linearity, utilizing fiberoptic cable runs for the sample and reference channels that are as shortas possible and held as rigid as possible (note that there are nosimilar restrictions on the fiber optic path common to the sample andreference channels), designing the diffraction grating to minimize thegeneration of stray light, designing the spectrograph to minimize thereflecting of stray light onto the photodetector, and using a modescrambler to ensure the uniform and reproducible imaging of light intothe spectrophotometer.

Photometric linearity is a particularly important component ofphotometric accuracy. The degree of photometric linearity is anindication of how accurately the light intensity signal is transformedinto a proportional electrical signal. Photometric linearity isgenerally a function of the inherent linearity of the photodetector andthe ability of the photoelectronics to produce an electrical signal thatis proportional to light intensity. Shifts in photometric linearity cancompromise prediction accuracy by distorting the absorbancedeterminations. For best results, the photometric response of thespectrophotometric process of the present invention should be linear tobetter than 0.9% of full scale intensity and preferably better than 0.2%of full scale intensity.

Methods to maintain or improve photometric linearity include designingthe photoelectronics to provide a signal that is proportional to thelight intensity incident on the detector and operating the photodiodearray detector outside of saturation. The response of photodiode arraydetectors, when operating near saturation, can become non-linear.Non-linearity due to photodiode array detector saturation can besubstantially reduced or eliminated by operating the photodiode arraydetector at less than 85% of full scale intensity.

The sampling time is the amount of time that polychromatic light isbeing launched through the sample and is generally quantified in termsof number of scales. Sampling time, in contradistinction to some of theother variables, is a controllable parameter, and can be adjusted foroptimum accuracy. While sampling time is easily controllable, it hasonly minimal effect on the accuracy of chemometric predictions when thenoise level in the photoelectronics is small. For example, predictionsfrom a single scan can be as accurate as those obtained from 1000 scansproviding the signal to noise ratio in the intensity spectra for thesample and reference channels exceeds 3000:1 over the spectral range ofinterest.

While sampling time can have minimal effect on prediction precision,sampling rate can have a substantial effect on chemometric predictionprecision. Where high sampling rates cause the electronics to reach peakspeeds, the photo-detection circuitry can cause photometricnon-linearity as described above. A suitable method for controllingsampling rate is to reduce the light source intensity and select aslower sampling rate that maintains photodiode array detector load fromabout 75% to about 85% of saturation in portions of the spectrum wherethe light intensity is greatest, in order to assure substantialphotometric linearity.

Changes in or adjustments to the optical path between thehigh-efficiency fiber optic switches should generally be monitored withparticular diligence. Changes made outside of the sample and referencepaths (i.e., outside of the high-efficiency fiber optic switches),generally result in minimal effects to prediction precision. This is dueto the fact that these changes are common to both the sample andreference channels and are essentially offsetting in the absorbancedetermination. Changes made within the sample or reference path canresult in prediction errors. These changes can include replacement ofthe high-efficiency fiber optic switches, replacement of a sample cell,or changes or movement in the fiber optic cables between thehigh-efficiency fiber optic switches. The mode scrambler and attenuatorutilized in the process of the present invention generally reduces theadverse effects of changes or adjustments made between thehigh-efficiency fiber optic switches. Where changes or adjustments aremade between the high-efficiency fiber optic switches, these changes canbe further offset by adjusting for changes in the background spectrum.The background spectrum can be determined by sampling an optically inertmaterial such as dry air, carbon tetrachloride, carbon disulfide, orfluorinert, and measuring the corresponding absorbance spectrum. Thetolerance of the spectrophotometric process of the present invention tochanges or adjustments to the optical path render the deviceparticularly suited to the rugged environment encountered in manymanufacturing facilities.

The repeatability of the high-efficiency fiber optic switches can affectthe precision of chemometric predictions by creating noise and causingor contributing to drift in the background spectrum. Noise can besimilarly produced by bending the fiber optic cables, by misaligning thefibers in the high-efficiency fiber optic switches, and accentuated ordiminished through use of different materials for the fiber optic cablesand switches. Generally, fiber optic switches and cable made fromfluorocarbon clad fibers perform better than glass clad fiber althoughboth are suitable for use in the present invention. Noise created fromall of the above described sources is substantially reduced through useof the mode scrambler. Noise or irreproducibilities in the measuredabsorbance spectra created from cycling the high-efficiency fiber opticswitches, can be reduced by a factor of 2, a factor of 3, and even asmuch as a factor of 5 by addition of the mode scrambling step of thepresent invention.

The sample temperature can affect prediction precision by changing thedensity of a sample and its resultant absorbance as described in aprevious equation. The density or inversely, the volume expansion forhydrocarbons common in petroleum refineries can often range from about0.02%/°F. (0.036%/°C.) to about 0.4%/°F. (0.72%/°C.) depending on theparticular hydrocarbon and the particular temperature range. Suitablemethods for mitigating the effects of sample temperature on predictionprecision include sample conditioning or maintaining the sampletemperature substantially constant, monitoring the sample temperatureand correcting for the effects mathematically, among other methods knownin the art. Monitoring the sample temperature and mathematicallycorrecting for temperature deviations from a target is simple andrequires a thermocouple and software steps. However, temperatureexpansion effects are generally not linear and correction models, undersome situations, may have to account for this non-linearity. Maintaininga constant sample temperature by sample conditioning can eliminateinaccuracies inherent to a temperature to volume expansion correlation,but generally requires a potentially awkward heat exchange step toensure proper temperature control. The sample temperature in a constantsample temperature device generally should be kept within a range ofabout 40° F. (22.2° C.) and more preferably within a range of about 20°F. (11.1° C.) for best results when monitoring the properties ofhydrocarbons. Sample temperature conditioning is particularly importantwhen effects other than thermal expansion, which introduce structure orother non-systematic changes in the spectra, cause the spectrum to lesspredictably change with temperature. In instances where hydrogen bondingis prevalent, such as in mixtures containing alcohols, sampletemperature ranges may need to be controlled to within 1° F. (0.6° C.),for best results.

The sample flowrate can effect prediction precision by the presence ofvariations in the amount of gas bubbles or in the magnitude of densityfluctuations in the sample stream. These variations, which can result inundesirable and uncompensated attenuation, can be caused or effected bythe level of turbulence at different flowrates. Where the sample isprovided by a slip-stream, flowrates can be adjusted to affect properoperation. Where the sample cell is an insertion-type probe, the sampleflow maya be reduced by modifying the sample cell itself or lowering thesupply line pressure. Generally, sample flowrate does not substantiallyeffect chemometric prediction precision as long as a suitable amount ofsample passes through the sample cell. Suitable sample velocities in thesample cell are generally less than about 10 ft/sec (3 meters/sec) andmore preferably less than about 1 ft/sec (0.3 meters/sec) for bestresults.

Environmental effects such as the introduction of non-sample relatedabsorbing components to the spectrum, can also effect predictionprecision. Non-sample related components such as hydrocarbon or water inthe form of smoke, hydrocarbon vapors, water vapor or humidity, canappear in the sample and/or reference spectra, introduced through pointsin spectrophotometric equipment where optical components are notentirely enclosed. Optical components having optical cavities where thepolychromatic light is exposed to the environment can include the lampsource, the attenuator, the high-efficiency fiber optic switches oroptical coupler, the sample cell, and the spectrograph. Non-samplerelated components which enter the system upstream of the first fiberoptic switch or downstream of the second fiber optic switch or opticalcoupler are generally less damaging to prediction precision since theseareas are common to both the reference and sample channels.

Non-sample related absorbing components can be minimized by purging ofthe particular cavities with a non-absorbing gas. Suitable non-absorbinggases can include, but are not limited to dry and hydrocarbon-freenitrogen, air, argon, helium, krypton, and radon. The preferrednon-absorbing purge gases are dry nitrogen and air. Purging of theoptical cavities can be performed on a continuous basis or can beperformed selectively. Where purging is performed continuously, thepurge rate should range from about 0.01 CFM (0.0003 CMM) to about 10 CFM(0.3 CMM) to insure sufficient optical cavity displacement. In aselective purge system, it is recommended that purging be performed eachtime an optical cavity is exposed to the environment. Upon exposure, theoptical cavity space should be replaced by 5 and preferably 10 times thevapor space of the non-absorbing gas at atmospheric conditions. Afterpurging with a suitable amount of non-absorbing purge gas, a positivepressure should be maintained on the sealed cavity in lieu ofmaintaining a constant purge rate. This method may be preferred wheresafety or non-absorbing gas usage are of particular concern and it isdesirable to minimize the volume of non-absorbing gas emitted to thespace surrounding the spectrophotometer.

The present invention provides an spectrophotometric process forobtaining spectral information and quantifying the physical propertiesof a sample that achieves superior chemometric prediction accuracy andis particularly reliable, durable, and stable over time. A process inaccordance with the principals of the present invention can generallyachieve a prediction accuracy for measuring physical properties ofbetter than plus or minus 1.0%, better than plus or minus 0.5%, and evenbetter than plus or minus 0.2%. A substantial proportion of thisprediction error can be, and is generally incurred from the primaryanalytical method used to quantify the property in building thechemometric model. Corrected for errors inherent in the primary methodused in building the chemometric model, the prediction accuracy can bebetter than plus or minus 0.1% and even better than 0.05%.

The spectrophotometric process of the present invention requiressubstantially no moving optical components other than thehigh-efficiency optical switches and is designed to factor out,substantially reduce, or mitigate imprecision in chemometric predictionsgenerally incurred by prior art devices and processes. When repairs arenecessary, such as the routine replacement of the light source orrepairs requiring movement of the device or fiber optics, the device andprocesses are designed to accommodate many of these repairs withoutrequiring costly recalibration of the instrument or redevelopment of thechemometric model.

The present invention provides a spectrophotometric process forobtaining spectral information and quantifying the physical propertiesof a sample that does not utilize fiber optic bundles or incur theinherent cost and imprecisions of fiber optic bundles. By eliminatinguse of fiber optic bundles, discernable artifacts in the absorbancespectra which are caused by small differences in the launching of lightinto and out of each fiber strand in the fiber optic bundle, and changesin these small differences with time, are eliminated. Elimination offiber optic bundles further reduces the need to recalibrate theanalyzer. Moreover, fiber optic bundles can cost from 10 to 100 timesmore than a single fiber optic strand and render some uses forspectrophotometric analysis cost prohibitive.

The present invention provides a spectrophotometric process forobtaining spectral information and quantifying the physical propertiesof a sample that efficiently and precisely measures the lighttransmitted through the sample and reference channels of the analyzerwith desensitized high-efficiency fiber optic switches and without otherdiverting devices that rely on the precise mechanical alignment of acritical optical component. The process of the present inventionutilizes high-efficiency fiber optic switches resulting in higher signalto noise ratio and improved photometric precision. The high-efficiencyfiber optic switches also generally reduce the time required to measurethe spectrum. The benefits of the high-efficiency fiber optic switchesare accommodated by a mode scrambling step, which compensates forvariability in the imaging of light from the launching to receivingports of the fiber optic switches. Addition of the mode scrambling stepreduces the magnitude of irreproducibilities featured in the spectrum,which are created from the fiber optic switches by a factor of 2, asmuch as a factor of 3, and even as much as a factor of 5.

The present invention provides a spectrophotometric process forobtaining spectral information and quantifying the physical propertiesof a sample that accurately, reproducibly, and expeditiously resolveswavelength at all relevant wavelengths and is not limited to sequentialwavelength measurement. The process of the present invention can measurethe spectrum of a sample at any or all of the relevant wavelengths ofinterest simultaneously, which results in a substantial increase in thespeed of spectral analysis. Faster spectral analysis results in fastersampling cycle rates, the analysis of a plurality of samples using thesame analyzer device, and/or the reduction or elimination of spectralartifacts caused by a rapid change in the physical or chemicalcomposition of the sample.

The present invention is described in further detail in connection withthe following examples, it being understood that the same are forpurposes of illustration and not limitation.

EXAMPLE 1

The integrity of the process of the present invention was tested bystudying the effects of apparatus and process variables on measurementaccuracy. The variables measured included the effects of filtering thespectra, changing optical components, wavelength precision, photometricprecision, photometric linearity, sampling time, changes in the opticalpath, high-efficiency fiber optic switch repeatability, sampletemperature, and sample flow rate.

The sample set utilized in the testing procedure consisted of 29 samplescontaining varying amounts of n-heptane, iso-octane, toluene,paraxylene, and n-decane. The range of concentrations of the componentsare described in Table 1.

                  TABLE 1                                                         ______________________________________                                        STATISTICS FOR THE SAMPLE SET                                                         Concentrations (Wt %)                                                 Constituent                                                                             Minimum      Maximum   Average                                      ______________________________________                                        n-Heptane 9.86         29.84     17.50                                        i-Octane  9.92         29.90     20.92                                        Toluene   29.96        49.95     38.75                                        p-Xylene  3.94         15.28     9.41                                         n-Decane  1.00         32.94     13.42                                        n-Alkanes*                                                                              14.85        47.10     30.91                                        ______________________________________                                         *wt % nAlkanes = wt % nHeptane + wt % nDecane                            

Prior to chemometric modeling and physical property prediction, theabsorbance spectra were filtered. Unless stated otherwise in succeedingexamples, regions outside of the third carbon-hydrogen stretchingovertone region (850-965 nanometers) were ignored. The absorbancespectra were differentiated once and then smoothed by convoluting theresulting spectra with a 1:2:1 distribution four times, unless otherwisenoted.

Four photometric measurement parameters were utilized to describe thetests performed. Transmittance (T) is the ratio of the light intensitiestransmitted by, and incident on the sample. Absorbance (ABS) is thenegative common logarithm of transmittance. Differential absorbance(dABS) is the first derivative of absorbance, computed using theformula:

    dABS=(T.sub.n-1 -T.sub.n+1)/T.sub.n

where T_(n) is the transmittance for the nth pixel. Transmittance,absorbance, and differential absorbance are specific to a particularwavelength. The gross or average efficiency of an optical component ismeasured in decibels (dB), which is ten times the common logarithm ofthe ratio of the transmitted to incident power. The average efficiencyis generally similar to absorbance except that the average efficiencypertains to a broad spectral range and is scaled differently thanabsorbance (i.e. E=-10 A).

Chemometric models were built for use in these examples using the CPACPLS program written by the Center for Process Analytical Chemistry atthe University of Washington (CPAC). To enhance accuracy, the spectrawere scaled by mean centering prior to modeling. Four latent variableswere used to model the concentrations of the five components in thesample set.

The effectiveness of the process of the present invention and thequality of the chemometric models were gauged by four statistics. Thefitted variance, calculated for the entire spectra and for eachcomponent of the sample set, is the fraction of the total variance inthe sample set described by the chemometric model. The fitted spectralvariance is a measure of how much of the information in the spectra ofthe sample set issued to model the properties of the sample set, and isdetermined as the percentage of the variance in the spectra that can bedescribed by the latent variables used to model the componentconcentrations.

The Standard Error of Estimation (SEE) is an indication of how well thechemometric model fits the sample set. The SEE for each component is ameasure of the inherent accuracy of the chemometric model and isdetermined as the standard deviation between the concentrationspredicted using the chemometric model and the laboratory measured valuesfor the 29 samples in the sample set.

The Standard Error of Prediction (SEP) is an indication of howaccurately the chemometric model predicts the properties of a test setof samples where the spectra and independent measurements of samplecomposition were not used in developing the chemometric model. The SEPis determined as the standard deviation between the concentrationspredicted using the chemometric model and independent laboratorymeasurements of sample composition.

The bias is the average offset or mean difference between the valuespredicted from the spectrophotometer using the chemometric model and theindependent laboratory measurements of composition for all samples inthe test set.

The effects of the filtering algorithm on the accuracy of chemometricprediction was determined by comparing the accuracy of prediction formodels built using the first derivative spectrum in the third overtoneband with binomial smoothing over 4 points, the first derivativespectrum in the third overtone band with binning over 4 points, thenormalized absorbance spectrum in the third overtone band with binomialsmoothing over 4 points where the absorbance across each of the measuredspectra was uniformly shifted to give a normalized absorbance of zero(0) near 820 nanometers, the second derivative spectrum in the thirdovertone band with binomial smoothing over 4 points, the firstderivative spectrum in the third overtone band with binomial smoothingover 8 points, the first derivative spectrum in the third overtone bandwith binning over 8 points, the first derivative spectrum in the thirdcombination band from about 965 nanometers to about 1070 nanometers withbinomial smoothing over 8 points, and the first derivative spectrum inthe third overtone band and the combination band with binomial smoothingover 8 points. The 4 point smooth was performed using 4 iterations withthe 1:2:1 weighting function and the 8 point smooth was performed using8 iterations with the 1:2:1 weighting function. All overtone andcombination bands pertain to vibrational modes which involve thestretching and bending of carbon-hydrogen bonds.

To build the chemometric models, 1000 scans of the spectra were obtainedand averaged. The SEE's for each component and model are shown in Table2, as are the SEPs obtained by averaging 1000 scans of the spectra ofunknowns and by using a single style scan to make the predictions of thecomponent concentrations.

                                      TABLE 2                                     __________________________________________________________________________    EFFECT OF FILTERING ON THE ACCURACY OF CHEMOMETRIC PREDICTIONS                (ERRORS ARE IN WT %)                                                          __________________________________________________________________________    Overtone Band     Overtone Band                                                                             Overtone Band                                                                             Overtone Band                       1st Derivative Spectrum                                                                         1st Derivative Spectrum                                                                   Normalized Absorbance                                                                     2nd Derivative Spectrum             4 pt. Smooth      4 pt. Bin   4 pt. Smooth                                                                              4 pt. Smooth                                  SEP         SEP         SEP         SEP                                       1000                                                                              SEP     1000                                                                              SEP     1000                                                                              SEP     1000  SEP                             SEE Scans                                                                             1 Scan                                                                            SEE Scans                                                                             1 Scan                                                                            SEE Scans                                                                             1 Scan                                                                            SEE Scans 1 Scan                    __________________________________________________________________________    n-Heptane                                                                           0.41                                                                              0.67                                                                              0.61                                                                              0.41                                                                              0.64                                                                              0.67                                                                              0.47                                                                              3.46                                                                              3.55                                                                              0.60                                                                              1.70  5.71                      i-Octane                                                                            0.16                                                                              0.21                                                                              0.31                                                                              0.16                                                                              0.21                                                                              0.36                                                                              0.16                                                                              0.43                                                                              0.43                                                                              0.16                                                                              0.36  0.80                      Toluene                                                                             0.15                                                                              0.12                                                                              0.15                                                                              0.15                                                                              0.11                                                                              0.20                                                                              0.15                                                                              0.51                                                                              0.50                                                                              0.17                                                                              0.29  0.85                      p-Xylene                                                                            0.11                                                                              0.13                                                                              0.15                                                                              0.11                                                                              0.12                                                                              0.20                                                                              0.18                                                                              1.78                                                                              1.74                                                                              0.14                                                                              0.56  1.90                      n-Decane                                                                            0.39                                                                              0.62                                                                              0.46                                                                              0.39                                                                              0.58                                                                              0.56                                                                              0.43                                                                              1.83                                                                              1.95                                                                              0.60                                                                              2.06  6.49                      n-Alkanes                                                                           0.17                                                                              0.18                                                                              0.21                                                                              0.17                                                                              0.17                                                                              0.19                                                                              0.18                                                                              1.68                                                                              1.63                                                                              0.20                                                                              0.40  0.86                      __________________________________________________________________________    Overtone Band     Overtone Band                                                                             Combination Band                                                                          Overtone and Combination Band       1st Derivative Spectrum                                                                         1st Derivative Spectrum                                                                   1st Derivative Spectrum                                                                   1st Derivative Spectrum             8 pt. Smooth      8 pt. Bin   8 pt. Smooth                                                                              8 pt. Smooth                                  SEP         SEP         SEP         SEP                                       1000                                                                              SEP     1000                                                                              SEP     1000                                                                              SEP     1000  SEP                             SEE Scans                                                                             1 Scan                                                                            SEE Scans                                                                             1 Scan                                                                            SEE Scans                                                                             1 Scan                                                                            SEE Scans 1 Scan                    __________________________________________________________________________    n-Heptane                                                                           0.41                                                                              0.68                                                                              0.61                                                                              0.41                                                                              0.59                                                                              1.00                                                                              0.43                                                                              2.80                                                                              8.79                                                                              0.40                                                                              0.75  0.87                      i-Octane                                                                            0.17                                                                              0.21                                                                              0.31                                                                              0.16                                                                              0.21                                                                              0.37                                                                              0.21                                                                              0.52                                                                              3.42                                                                              0.16                                                                              0.21  0.41                      Toluene                                                                             0.15                                                                              0.11                                                                              0.15                                                                              0.15                                                                              0.12                                                                              0.23                                                                              0.43                                                                              1.73                                                                              3.24                                                                              0.15                                                                              0.10  0.14                      p-Xylene                                                                            0.11                                                                              0.12                                                                              0.15                                                                              0.11                                                                              0.13                                                                              0.22                                                                              0.48                                                                              1.84                                                                              4.13                                                                              0.11                                                                              0.12  0.20                      n-Decane                                                                            0.39                                                                              0.63                                                                              0.46                                                                              0.39                                                                              0.55                                                                              0.84                                                                              0.43                                                                              2.66                                                                              7.28                                                                              0.38                                                                              0.69  0.71                      n-Alkanes                                                                           0.17                                                                              0.18                                                                              0.21                                                                              0.17                                                                              0.16                                                                              0.21                                                                              0.17                                                                              0.24                                                                              1.65                                                                              0.17                                                                              0.18  0.22                      __________________________________________________________________________

The chemometric models developed from first-derivative spectra weregenerally more accurate than models built from second-derivative andnormalized absorbance spectra. Using the combination band alone ortogether with the overtone band similarly reduced the accuracy of themodels because of noise in the spectra used to build the model. Binomialsmoothing and binning provided similar accuracy as did increasing thesmoothing iterations from 4 pints to 8 points.

EXAMPLE 2

The effects of changing optical components, consistent with utilizationof the process of the present invention in a field or manufacturingenvironment, were measured and compared to the precision ofspectrophotometric processes utilizing components more typical of thoseused in a laboratory environment. The base spectrophotometer utilized acommercial light source (model 780, manufactured by Newport), a cuvetteholder which held cuvettes having a 10 cm path length, no fiber opticswitches, no long run of fiber optics or mode scrambler, and noattenuation means. The reference spectra was measured by emptying thecuvette holder.

The first optical component change was use of a different light source.The light sources generally have three major components: a lamp, a powersupply, and optics for focusing the light into the fiber optics. The newlight source was a tungsten-halogen type (Welch-Allyn No. 998319-15)with a lens end of 7 mm D×14 mm F. The filament size was 0.5 mm×1.2 mm.The lamp was designed to run at 5.0 V, 1.8 A, and 2900° K. colortemperature. The nominal life was 6000 hours. The power supply (AdtechAPS 5-3) had an output of 5.0 V at 3 A. The voltage regulation was 0.1%for a 10% change in line or load. The light from the lamp was focusedinto the fiber optic cable with a biconvex lens of 10 mm D×10 mm F. AnRG780 filter, manufactured by Corning, prevented visible light fromentering the spectrophotometer. The glass optics were mounted in a steelbody and held in place with rulon bearings. The fiber optic was cementedinto a modified SMA connector, having the coupling nut removed and theend machined to 3/16 in and the modified connector and fiber optic wasinserted in the same steel body to provide a means for launching lightfrom the source into the fiber optic cable. In Table 3, the performanceof the process utilizing the modified light source is compared to thespectrophotometric processes utilizing components more typical of thoseused in a laboratory environment.

The second component change was the addition of two high-efficiencyfiber optic switches for use in self-referencing the spectrophotometerto reduce the effects of drift. The high-efficiency fiber optic switcheswere latching type in a 1×2 configuration (Dicon S-12-L-200). The actualswitching was performed by moving the common fiber between the twoports. The coupling efficiency, measured as the light loss across thefiber junction, was 1 db and the minimum switching time was 10 msec or100 cycles per second. The switches drew 150 mA at 5.0 V. Theperformance of the process utilizing high-efficiency fiber opticswitches is also noted in Table 3.

The third component change was the combination of the high-efficiencyfiber optic switches described in the second component change and a longrun of fiber optic cable between the downstream high-efficiency fiberoptic switch and the spectrograph, for serving as mode scrambling means.The fiber optic cable had a core diameter of 200 μm, a 230 μmfluorocarbon cladding, and a 0.3 NA (Ensign-Bickford HCP-M0200T-06). Thecore was low-OH silica and the attenuation was of less than 10 db perkilometer of fiber optic cable across the spectral range of from 800nanometers to 1100 nanometers. The length of fiber optic cable used inthe long run was about 700 feet. The performance of the processutilizing high-efficiency fiber optic switches and mode scrambling meansis also noted in Table 3.

The fourth component change was the combination of the new light sourcedescribed in the first component change, the high-efficiency opticswitches and mode scrambling means described in the third componentchange, and the addition of a sample cell. Light was routed into and outof the sample cell with two fiber optic cables, mounted side by side.Light from the first fiber optic cable was collimated with an achromaticdoublet lens, passed through the sample, struck a reflection device,passed through the sample again, and was reimaged into the second fiberoptic cable with a second achromatic doublet lens. Note that acollimating lens changes the conical distribution of light rays emittedfrom the fiber optic cable into a set of collimated or parallel rays.The achromatic doublets were 6.25 mm D×12.5 mm F and were mounted to thesteel body of the sample cell with Teflon bearings. The reflectiondevice was a retroreflector or corner cube that was slip-fit into thesteel body, had a 15 mm D, and was designed to reflect light three timesprior to passing the light through the sample the second time. Twowindows, 25 mm D×6 mm T, isolated the sample from the optics. Thewindows were sealed on O-rings. The total path length through the sample(L), was about 10 cm, which provided a maximum absorbance of roughly 0.5ABS in the 800 nanometer to 1100 nanometer range for typical hydrocarbonmaterials. The optical attenuation in the cell, including connectorlosses, was 7 db. Based on a 4% loss at each glass/air interface, thesample cell attenuation was expected to be about 3 db. The performanceof the process utilizing the modified light source, high-efficiencyfiber optic switches, extended fiber optics, and sample cell is alsonoted in Table 3.

                  TABLE 3                                                         ______________________________________                                        EFFECT OF OPTICAL COMPONENTS ON THE                                           ACCURACY OF CHEMOMETRIC PREDICTIONS                                           (ALL ERRORS ARE IN WT %)                                                      ______________________________________                                        Type of Optical                                                               Components                                                                    Lamp         -       +       -     -     +                                    Switches     -       -       +     +     +                                    Sample Cell  -       -       -     -     +                                    Long Fiber   -       -       -     +     +                                    Fitted Variance (%)                                                           Spectral     99.81   99.88   98.44 99.79 99.80                                n-Heptane    99.34   99.58   99.31 99.13 99.53                                i-Octane     99.95   99.83   99.91 99.81 99.96                                Toluene      99.98   99.78   99.98 99.74 99.99                                p-Xylene     99.90   99.93   99.87 99.91 99.92                                n-Decane     99.77   99.83   99.80 99.67 99.81                                n-Alkanes    99.99   -       -     99.97 99.99                                SEE Model (Wt %)                                                              n-Heptane    0.52    0.42    0.54  0.60  0.45                                 i-Octane     0.14    0.26    0.20  0.27  0.13                                 Toluene      0.09    0.31    0.08  0.33  0.06                                 p-Xylene     0.12    0.10    0.13  0.11  0.11                                 n-Decane     0.47    0.40    0.43  0.55  0.42                                 n-Alkanes    0.09    -       -     0.15  0.09                                 SEP (Wt %)                                                                    n-Heptane    1.10    0.75    1.33  0.96  0.88                                 i-Octane     0.30    0.41    0.42  0.34  0.27                                 Toluene      0.14    0.37    0.17  0.45  0.10                                 p-Xylene     0.12    0.15    0.17  0.14  0.14                                 n-Decane     0.91    0.57    1.08  0.82  0.73                                 n-Alkanes    0.16    -       -     0.19  0.16                                 ______________________________________                                         (-) DENOTES TYPICAL LABORATORY OPTICAL COMPONENTS                             (+) MODIFIED OR ADDITIONAL COMPONENTS                                    

Table 3 illustrates that chemometric predictions obtained utilizing thefield or manufacturing environment-enhanced optical components providedsimilar precision to the laboratory device. Enhancing thespectrophotometer having high-efficiency fiber optic switches with thelong fiber optic run between the second fiber optic switch and thespectrograph improved the accuracy of the predictions by acting as amode scrambler, which improved precision in the spectral measurements.Adding the sample cell with fixed optics provided further predictionaccuracy improvements.

EXAMPLE 3

The effect of uniform shifts in the wavelength axis (x-axis) of theintensity spectra on the accuracy of chemometric predictions wasdetermined through software manipulations. Chemometric models were builtand then the composition of samples were predicted after uniformlyshifting the spectra. Shifts in the wavelength axis can be caused bymechanical vibrations or shocks or changes in the temperature of thespectrograph. The uniform shifts in wavelength axis were simulatedthrough software by first assuming that the intensity was uniformlydistributed across each pixel. Each pixel was about 0.25 nanometerswide, and the overtone band consisted of 400 pixels. The precisionresults of various shifts in the wavelength axis are illustrated inTable 4.

                  TABLE 4                                                         ______________________________________                                        EFFECT OF SHIFTING THE WAVELENGTH AXIS                                        ON THE ACCURACY OF CHEMOMETRIC                                                PREDICTIONS                                                                          Shift in Fraction of Pixel                                                       0.01  0.03    0.10    0.30  1.00                                    ______________________________________                                        Bias (Wt %)                                                                   n-Heptane                                                                              0.00   0.16    0.49  1.64  4.91  16.36                               i-Octane 0.00   -0.02   -0.07 -0.23 -0.70 -2.35                               Toluene  0.00   0.02    0.07  0.23  0.69  2.29                                p-Xylene 0.00   -0.03   -0.08 -0.28 -0.85 -2.81                               n-Decane 0.00   -0.13   -0.40 -1.34 -4.03 -13.46                              n-Alkanes                                                                              0.00   0.03    0.09  0.29  0.87  2.91                                SEP (Wt %)                                                                    n-Heptane                                                                              0.41   0.44    0.65  1.72  5.04  16.77                               i-Octane 0.16   0.16    0.18  0.29  0.75  2.44                                Toluene  0.15   0.15    0.17  0.28  0.72  2.35                                p-Xylene 0.11   0.11    0.14  0.31  0.89  2.91                                n-Decane 0.38   0.40    0.56  1.43  4.15  13.79                               n-Alkanes                                                                              0.17   0.17    0.19  0.34  0.91  2.98                                ______________________________________                                    

Table 4 illustrates that shifting the wavelength axis or spectrum by0.03 pixels provides a noticeable increase in the error in prediction.Shifting the spectrum 0.10 pixels or 0.03 nanometers provides larger andmore significant inflation in the error of prediction. Shifts of a fullpixel in wavelength axis can provide chemometric predictions that maynot be accurate enough to utilize for some process services.

The effects of uniform shifts in the wavelength axis caused bytemperature variations in the spectrograph were determined by building achemometric model at ambient temperature (around 70° F. or 21.1° C.) andheating the spectrograph to about 120° F. (48.9° C.) for sampleprediction. In a separate experiment, predictions were made after theambient temperature was decreased by 10° F. (5.6° C.) to about 60° F.(15.6° C.). The precision results from the ambient temperature inducedwavelength axis shifts are illustrated in Table 5.

                  TABLE 5                                                         ______________________________________                                        EFFECT OF WAVELENGTH DRIFT ON THE                                             ACCURACY OF CHEMOMETRIC PREDICTIONS.                                          DRIFT WAS INDUCED BY HEATING THE                                              SPECTROGRAPH AND BY CHANGES                                                   IN THE AMBIENT TEMPERATURE                                                           Heat          Reduce                                                          Spectrograph  Ambient Temperature                                             Before After      Before   After                                       ______________________________________                                        Bias Wt %                                                                     n-Heptane                                                                              0.04     -5.72      -0.32  1.34                                      i-Octane -0.03    0.09       0.06   -0.36                                     Toluene  -0.05    0.72       -0.04  0.46                                      p-Xylene 0.08     0.17       -0.06  -0.26                                     n-Decane -0.05    4.82       0.35   -1.18                                     n-Alkanes                                                                              -0.01    -0.90      0.03   0.16                                      SEP (Wt %)                                                                    N-Heptane                                                                              0.24     6.70       0.50   1.55                                      i-Octane 0.10     0.56       0.12   0.41                                      Toluene  0.09     1.15       0.04   0.51                                      p-Xylene 0.15     0.66       0.09   0.31                                      n-Decane 0.20     5.70       0.52   1.40                                      n-Alkanes                                                                              0.04     1.05       0.04   0.18                                      ______________________________________                                    

Table 5 illustrates that changes in ambient temperature producesubstantial errors in the precision of predictions. Extrapolating fromTable 4, increasing the ambient temperature to 120° F. (48.9° C.)created a shift of about 0.4 pixels. The results of both the ambienttemperature increase and decrease cases indicated that the wavelengthaxis can shift about 0.01 pixels per °F. or 0.02 pixels per °C. changein ambient temperature. These tests further indicate that chemometricprediction accuracy can be gained by maintaining the spectrograph at aconstant temperature or correcting for the ambient temperature changesmathematically. For example, a thermocouple can be installed in thespectrograph to measure the ambient temperature and utilize its signalto calculate and adjust for the shift in wavelength axis.

EXAMPLE 4

The effects of reducing the photometric precision of the spectrograph onthe accuracy of chemometric predictions was similarly determined throughsoftware manipulations. Variables that affect photometric precision caninclude stray light in the spectrograph, the resolution of the intensityaxis, noise, and non-linearity in the photo-electronics. Stray light canbe caused by scattering from the fixed diffraction grating and wasmeasured at about 1 part in 3000 for the spectrograph used. The effectof increasing stray light over this base level was simulated by adding aconstant offset to the baseline of the intensity spectra. The effects ofincreasing stray light on photometric precision are illustrated in Table6.

                  TABLE 6                                                         ______________________________________                                        THE EFFECT OF STRAY LIGHT                                                     ON THE ACCURACY OF CHEMOMETRIC                                                PREDICTIONS                                                                   Imposed Stray Light                                                           (Percent of Full-Scale Intensity)                                                    0    0.1      0.3      1.0    3.0                                      ______________________________________                                        Bias Wt %                                                                     n-Heptane                                                                              0.00   0.11     0.34   1.13   3.39                                   i-Octane 0.00   0.06     0.18   0.61   1.90                                   Toluene  0.00   -0.01    -0.03  -0.11  -0.34                                  p-Xylene 0.00   -0.06    -0.19  -0.64  -1.98                                  n-Decane 0.00   -0.10    -0.30  -0.99  -2.97                                  n-Alkanes                                                                              0.00   0.01     0.04   0.14   0.42                                   SEP (Wt %)                                                                    Heptane  0.41   0.42     0.53   1.23   3.51                                   i-Octane 0.16   0.17     0.25   0.67   2.03                                   Toluene  0.15   0.15     0.16   0.22   0.52                                   p-Xylene 0.11   0.13     0.22   0.67   2.04                                   n-Decane 0.38   0.39     0.49   1.09   3.10                                   n-Alkanes                                                                              0.17   0.17     0.18   0.30   0.78                                   ______________________________________                                    

Table 6 illustrates that stray light levels up to 0.1% of the full-scaleintensity do not substantially affect the accuracy of chemometricpredictions in this instance where the maximum absorbance level wasabout 0.5 units. In spectrophotometers having longer sample path lengthsor analyzing substances having stronger absorbances, the effects ofstray light can be more severe.

The effect of the resolution of the intensity axis (y-axis) on theaccuracy of chemometric predictions, which generally depends on thesignal-to-noise ratio of the photo-electronics, sampling time, and theresolution of the analog-to-digital converter, was simulated by roundingoff the intensity measurement in the spectra used to build and test thechemometric models. The resolution of the intensity axis was given inbits n, where the resolution is one part in 2^(n). The fewer the numberof bits, the greater the round off error prior to modeling. The resultsof reduction in resolution of the intensity axis on prediction accuracyare illustrated in Table 7.

                  TABLE 7                                                         ______________________________________                                        EFFECT OF THE RESOLUTION OF THE INTENSITY                                     AXIS ON THE ACCURACY OF CHEMOMETRIC                                           PREDICTIONS                                                                              Resolution of Intensity Axis (bits)                                           15    12      10      9     8                                      ______________________________________                                        Fitted Variance (%)                                                           Spectral     99.95   99.89   99.12 97.05 89.20                                n-Heptane    99.61   99.63   99.13 98.09 95.94                                i-Octane     99.93   99.93   99.91 99.90 99.80                                Toluene      99.94   99.94   99.96 99.94 99.86                                p-Xylene     99.41   99.90   99.89 99.89 99.62                                n-Decane     99.84   99.84   99.70 99.39 98.58                                n-Alkanes    99.96   99.97   99.96 99.95 99.93                                SEE Model (Wt %)                                                              n-Heptane    0.41    0.40    0.61  0.90  1.30                                 i-Octane     0.16    0.16    0.19  0.20  0.28                                 Toluene      0.15    0.15    0.13  0.15  0.24                                 p-Xylene     0.11    0.12    0.12  0.12  0.23                                 n-Decane     0.39    0.38    0.53  0.73  1.25                                 n-Alkanes    0.17    0.17    0.18  0.20  0.23                                 SEP (Wt %)                                                                    n-Heptane    0.86    0.78    0.87  1.43  4.45                                 i-Octane     0.34    0.33    0.34  0.36  0.84                                 Toluene      0.19    0.19    0.18  0.20  0.57                                 p-Xylene     0.11    0.12    0.14  0.29  0.85                                 n-Decane     0.65    0.61    0.80  1.26  3.82                                 n-Alkanes    0.28    0.26    0.20  0.24  0.69                                 ______________________________________                                    

Table 7 illustrates that a resolution of 10 bits, which corresponds to0.0004 to 0.001 ABS, is suitable for intensity axis resolution in thisinstance where the peak absorbance was about 0.5. On thespectrophotometer utilized, this accuracy was obtained with one scan ofthe spectrum, which was completed in 20 msec. More resolution may berequired for spectcrophotometers having longer sample pathlengths oranalyzing more strongly-absorbing materials.

The effect of spectral noise on the accuracy of chemometric predictionswas determined by artificially imposing noise levels of differentmagnitudes onto the intensity spectra used for building and testing thechemometric models. The noise level at each point in the spectrum wasselected using a random number generator operating within theconstraints of the overall magnitude of the noise level desired. Noisecan be produced in a spectrophotometer by the imprecise repositioning ofthe fiber optics in the switches and in the photoelectronics and thelight source by electromagnetic interference and other electricaldisturbances. The peak to peak noise level was described as a percent ofthe full-scale intensity range of the photodiodes. The results ofincreased noise on prediction accuracy are illustrated in Table 8.

                  TABLE 8                                                         ______________________________________                                        THE EFFECT OF IMPOSED NOISE ON THE                                            ACCURACY OF CHEMOMETRIC PREDICTIONS                                                  Imposed Peak-to-Peak Noise Level                                              (Percent of Full-Scale Intensity)                                             0    0.03     0.1      0.3    1.0                                      ______________________________________                                        Bias Wt %                                                                     n-Heptane                                                                              0.00   -0.02    0.10   -0.36  0.37                                   i-Octane 0.00   0.00     0.00   0.10   -0.14                                  Toluene  0.00   -0.00    0.01   0.00   -0.06                                  p-Xylene 0.00   0.00     -0.03  -0.03  0.12                                   n-Decane 0.00   0.02     -0.08  -0.29  -0.30                                  n-Alkanes                                                                              0.00   0.00     0.02   -0.07  0.07                                   SEP (Wt %)                                                                    Heptane  0.41   0.48     0.59   2.23   6.24                                   i-Octane 0.16   0.18     0.21   0.52   1.35                                   Toluene  0.15   0.16     0.16   0.34   0.91                                   p-Xylene 0.11   0.12     0.15   0.48   1.19                                   n-Decane 0.38   0.43     0.50   1.81   4.97                                   n-Alkanes                                                                              0.17   0.18     0.19   0.47   1.34                                   ______________________________________                                    

Table 8 illustrates that noise levels between 0.03% and 0.1% of fullscale can effect the accuracy of the chemometric predictions. Over mostof the test, the spectrophotometer utilized in the process of thepresent invention had a base noise level of 0.02% of full scale and wasgenerally caused by the imprecise repositioning of the fiber cables inthe switches.

EXAMPLE 5

The photometric linearity, or how accurately the light intensity signalwas transformed into a proportional electrical signal, and the effectsof linearity on the accuracy of chemometric predictions were determinedfor the process of the present invention. The linearity was tested bymeasuring the spectra of mixtures of benzene and cyclohexane with knowncomposition. This system was used because there is substantially nohydrogen bonding between molecules, the spectra of the two compoundshave very little overlap, and both components are available in highpurity (99⁺ wt %).

Since absorbance depends on the sample density, non-ideal mixinggenerally appears as a non-linearity in the absorbance and should beaccounted for. The mixing of benzene and cyclohexane is non-idealwhereby the volume increase upon mixing can be approximated by:

    ΔV.sub.mix =0.03×W.sub.b ×(1-W.sub.b)

where W_(b) is the weight fraction of benzene in cyclohexane. Thedensities of mixtures of benzene and cyclohexane are illustrated inTable 9.

                  TABLE 9                                                         ______________________________________                                        DENSITIES OF MIXTURES OF BENZENE AND                                          CYCLOHEXANE                                                                   (g/cc @ 60° F.)                                                        Wt % Benzene                                                                            Measured Density                                                                            Ideal Density                                                                            ΔV.sub.mix (%)                       ______________________________________                                         0.00     0.7766        --         0.0                                        25.02     0.7918        0.7995     0.5                                        49.84     0.8170        0.8237     0.8                                        74.91     0.8453        0.8496     0.4                                        100.00    0.8772        --         0.0                                        ______________________________________                                    

Spectra of mixtures of benzene and cyclohexane, with known composition,were measured using spectrophotometers having two different siliconarray detectors driven by different electronics (Spectrophotometers Aand B). Spectrophotometer A utilized a Reticon S-Series photodiode arraywhile Spectrophotometer B utilized a Reticon SB Series photodiode array.Chemometric models of composition were built for each spectrophotometerutilizing one latent variable and the SEE for reach spectrophotometerand chemometric model determined. The SEP was also determined for a testset measured on Spectrophotometer A. The results of the evaluation ofthe absorbance linearity for Spectrophotometer A are illustrated inTable 10.

                  TABLE 10                                                        ______________________________________                                        EVALUATION OF THE ABSORBANCE LINEARITY                                        OF SPECTROPHOTOMETER A USING                                                  SOLUTIONS OF BENZENE AND CYCLOHEXANE                                                   Errors of              Errors of                                     Wt %     Estimation    Wt %     Prediction                                    Benzene  (WT %))       Benzene  (WT %)                                        ______________________________________                                         0.0      0.34         0.05     0.31                                          10.3      0.35         1.04     0.27                                          20.4     -0.10         2.02     0.25                                          30.4      0.06         5.03     0.17                                          39.6     -0.48         10.04    0.06                                          50.0     -0.41         19.99    -0.22                                         59.8     -0.17         50.03    -0.35                                         69.7     -0.26         100.00   0.69                                          79.6     -0.06                                                                89.7      0.16                                                                100.0     0.55                                                                SEE       0.33         SEP      0.36                                          ______________________________________                                    

Spectrophotometer B was further tested with and without isolating thespectrograph of Spectrophotometer B from vibrational effects. Theoperation of a nearby fume hood caused slight vibration in thelaboratory bench. The results of the evaluation for Spectrophotometer Bare illustrated in Table 11.

                  TABLE 11                                                        ______________________________________                                        EVALUATION OF THE ABSORBANCE LINEARITY                                        OF SPECTROPHOTOMETER B USING                                                  SOLUTIONS OF BENZENE AND CYCLOHEXANE                                                    Errors of Estimation (Wt %)                                         Wt %        Without Vibration                                                                          With Vibration                                       Benzene     Isolation    Isolation                                            ______________________________________                                         0.0         0.21         0.18                                                10.6         0.32         0.30                                                20.2         0.02         0.04                                                29.5        -0.12        -0.16                                                40.0        -0.16        -0.18                                                50.3        -0.20        -0.18                                                59.8        -0.50        -0.28                                                68.6        -0.11        -0.17                                                80.1         0.03         0.01                                                91.0        -0.04        -0.03                                                100.0        0.56         0.48                                                SEE          0.30         0.25                                                ______________________________________                                    

Tables 10 and 11 illustrate that the observed non-linearities of -0.7 wt% to -0.9 wt %, determined as the average SEE of the 0.0 wt % benzeneand 100.0 wt % benzene cases less the SEE of the 50 wt % benzene case,are very close to those expected from the non-ideal mixing of theconstituents. The apparent non-linearities of the photo-response forboth Spectrophotometers A and B were less than 0.1 wt % at 50 wt %benzene, or 0.2%. Since the SEEs were similar for the cases wherevibration was and was not isolated, Spectrophotometer B was notsensitive to vibrations. In this instance, the maximum absorbance wasabout 0.7 units. In instances where the maximum absorbance is lower, thenon-linearity may be less severe.

Spectrophotometer A was further tested for photometric linearity, thatis, linearity in the intensity measurements, by building chemometricmodels for benzene, changing the photometric range, and predicting thecompositions of unknowns. The photometric range was varied by changingthe intensity of the light source, the sampling rate, and by inserting a700 ft. (211.6 meter) long run of 200 micron fiber optic cable into theoptical path. The SEPs for each case were determined with and withoutrespect to the control samples used to uniquely determine the errorscaused by photometric non-linearity. The results of the evaluation ofphotometric linearity are illustrated in Table 12.

                                      TABLE 12                                    __________________________________________________________________________    TEST OF PHOTOMETRIC LINEARITY BY VARYING THE SOURCE                           INTENSITY AND SCAN RATE, AND BY INSERTING 200 m OF                            FIBER INTO THE OPTICAL PATH (ERRORS OF PREDICTION ARE                         FOR WT % BENZENE IN CYCLOHEXANE)                                              __________________________________________________________________________    Source Intensity                                                                           80 60 40 20 80 80 80  80*                                        (% Full-Scale)                                                                Sampling Rate                                                                              25 25 25 25 25 50 100 25                                         (Spectra/Sec)                                                                 Model w/1 Latent Vector                                                       SEP          0.51                                                                             0.51                                                                             0.50                                                                             0.52                                                                             0.52                                                                             0.53                                                                             0.54                                                                              0.59                                       SEP wrt Control                                                                            0.00                                                                             0.07                                                                             0.13                                                                             0.21                                                                             0.00                                                                             0.06                                                                             0.05                                                                              0.20                                       Model w/2 Latent Vector                                                       SEP          0.32                                                                             0.31                                                                             0.33                                                                             0.36                                                                             0.30                                                                             0.34                                                                             0.33                                                                              0.38                                       SEP wrt Control                                                                            0.00                                                                             0.07                                                                             0.13                                                                             0.21                                                                             0.00                                                                             0.06                                                                             0.05                                                                              0.23                                       __________________________________________________________________________     *200 m of fiber optic inserted into optical path                         

Table 12 illustrates that when the photometric range was reduced up tofour-fold by reducing the source intensity or sampling rate, thepredictions of concentration were accurate to 0.2 wt % with respect tothe controls. When the spectral range was attenuated non-uniformly byinserting the long run of fiber optic cable, similar accuracy wasobtained. The results of the photometric linearity tests indicated thatthe photometric response of the process of the present invention islinear to better than 0.2% of full scale. For best results, the scanrate of the photoarray detector was increased to maintain the maximumintensity on any of the detectors below 85% of the saturation value.

EXAMPLE 6

The effect of sampling time and sampling rate on the accuracy ofchemometric predictions was determined for the process of the presentinvention. A chemometric model was built using spectra averaged over1000 scans, then the spectra of unknowns were measured by averaging avariable number of scans. The effect of varying sampling time was variedby changing the number of scans from 1 to 1000 scans and thendetermining the Bias and SEP. The results are illustrated in Table 13.

                  TABLE 13                                                        ______________________________________                                        EFFECT OF SAMPLING TIME ON THE ACCURACY                                       OF CHEMOMETRIC PREDICTIONS                                                           # Scans at 50 Spectra/Sec                                                     1000    100       10        1                                          ______________________________________                                        Bias Wt %                                                                     n-Heptane                                                                              0.43      0.30      0.20    0.04                                     i-Octane -0.01     -0.07     -0.03   0.02                                     Toluene  -0.09     -0.04     -0.19   -0.02                                    p-Xylene 0.05      -0.04     0.01    -0.05                                    n-Decane -0.40     -0.18     -0.14   -0.02                                    n-Alkanes                                                                              0.03      0.12      -0.06   0.03                                     SEP (Wt %)                                                                    Heptane  0.67      0.54      0.37    0.60                                     I-Octane 0.21      0.32      0.26    0.31                                     Toluene  0.12      0.20      0.42    0.15                                     p-Xylene 0.13      0.14      0.12    0.15                                     n-Decane 0.60      0.46      0.37    0.46                                     n-Alkanes                                                                              0.17      0.24      0.15    0.21                                     ______________________________________                                    

Table 13 illustrates that predictions from a single scan were generallyas accurate as those obtained by averaging 1000 scans. This was expectedfrom previous results because the noise in the spectra is random and isabout 0.1% of the intensity scale for a single scan. As was shown inTable 7, this level of random noise has minimal effect on the accuracyof the chemometric predictions.

EXAMPLE 7

The effects of changes in the optical path on the accuracy ofchemometric predictions were determined by making changes in the opticalpath both inside and outside the high-efficiency fiber optic switches.The fiber optic path was first changed by remaking connections in theoptical path that was common to the sample and reference channels. Thetest was performed on a spectrophotometer without an attenuator or amode scrambler. The results of changing the optical path as describedabove are illustrated in Table 14.

                  TABLE 14                                                        ______________________________________                                        EFFECT OF CHANGES IN THE OPTICAL PATH                                         COMMON TO THE SAMPLE AND REFERENCE                                            CHANNELS ON THE ACCURACY OF CHEMOMETRIC                                       PREDICTIONS                                                                                     Remake Connection                                                      Control                                                                              Outside Switches                                            ______________________________________                                        Bias (Wt %)                                                                   n-Heptane    0        -0.38                                                   i-Octane     0.01     -0.21                                                   Toluene      0        -0.15                                                   p-Xylene     -0.07    0.50                                                    n-Decane     0.06     0.23                                                    n-Alkanes    0.06     -0.15                                                   SEP (Wt %)                                                                    n-Heptane    0.52     0.64                                                    i-Octane     0.10     0.23                                                    Toluene      0.11     0.18                                                    p-Xylene     0.07     0.57                                                    n-Decane     0.56     0.53                                                    n-Alkanes    0.06     0.20                                                    ______________________________________                                    

Table 14 illustrates that remaking a fiber optic connection outside thefiber optic switches had little effect on prediction accuracy. This isbecause measuring the intensity spectra for the sample and referencechannels compensates for aberrations in the optics that are common tothe two channels. Altering the optics in the path that is unique to thesample or reference channel has larger effects on prediction accuracy.

The fiber optic path between the two high-efficiency fiber opticswitches was then changed by moving the fiber optic cable in the samplechannel, swapping fiber optic switches, and inserting a new sample cell.The apparent absorbance spectrum was then adjusted for the changes inthe background spectrum or sample path length. The relative path lengthfor the two sample cells was determined by measuring the apparentabsorbance of benzene, subtracting the background spectrum, and ratioingthe difference between cells. These tests were also performed on aspectrophotometer without an attenuator or a mode scrambler. The resultsof changing the optical path as described above are illustrated in Table15.

                                      TABLE 15                                    __________________________________________________________________________    EFFECT OF CHANGING THE OPTICAL PATH BETWEEN THE SAMPLE AND REFERENCE          CHANNELS ON THE ACCURACY OF CHEMOMETRIC PREDICTIONS                                    MOVE FIBER OPTIC  SWAP FIBER SWITCH SWAP SAMPLE CELL                                      AFTER             AFTER             AFTER                               AFTER COR-        AFTER COR-        AFTER COR-                          INITIAL                                                                             CHANGE                                                                              RECTION                                                                             INITIAL                                                                             CHANGE                                                                              RECTION                                                                             INITIAL                                                                             CHANGE                                                                              RECTION              __________________________________________________________________________    BIAS (WT %)                                                                   n-Heptane                                                                              0.07  0.74  -0.08 0.04  8.11  -0.33 0.04  -2.18 -0.67                i-Octane -0.04 -0.13 0.09  -0.03 -0.24 0.17  -0.03 0.07  0.36                 Toluene  0.03  0.07  -0.16 -0.05 -0.11 0.22  -0.05 0.00  0.01                 p-Xylene 0.01  -0.30 0.06  0.08  0.40  -0.38 0.08  0.02  -0.49                n-Decane -0.12 -0.39 0.19  -0.05 -8.16 0.37  -0.05 2.14  0.84                 n-Alkanes                                                                              -0.05 0.35  0.11  0.01  -0.04 0.04  0.01  -0.04 0.18                 SEP (wt %):                                                                   n-Heptane                                                                              0.55  0.96  0.54  0.24  9.11  0.98  0.24  2.24  0.88                 i-Octane 0.46  0.46  0.44  0.10  0.30  0.22  0.10  0.20  0.42                 Toluene  0.42  0.33  0.37  0.09  0.12  0.24  0.09  0.14  0.08                 p-Xylene 0.22  0.38  0.20  0.15  0.46  0.43  0.15  0.12  0.56                 n-Decane 0.67  0.76  0.67  0.20  9.16  0.84  0.20  2.19  1.02                 n-Alkanes                                                                              0.31  0.52  0.36  0.04  0.19  0.20  0.04  0.05  0.22                 __________________________________________________________________________

Table 15 illustrates that moving the fiber optics between thehigh-efficiency fiber optic switches and swapping a fiberhigh-efficiency optic switch or sample cell with a similar type,produced sizeable prediction errors. Where the fiber optic was moved,adjusting for changes in the background spectrum nearly eliminated thebias in the measurement. Where the high-efficiency fiber optic switchesand sample cells were swapped, the adjustment for changes in thebackground spectrum reduced the bias significantly, but not entirely.

The addition of an attenuator and mode scrambler to thespectrophotometer made the predictions less sensitive to changes in thebackground spectrum. In the Example, the attenuator had two plano-convexlenses for collimating the light between the launching and receivingfiber optic cables in the reference channel. The background spectrum wasmeasured by filling the sample cell with carbon tetrachloride. The samespectra for each of the 29 samples were used for modeling andprediction, except that the background correction was applied to eachspectrum before modeling the composition, but not applied when thespectra were used to predict composition. The results are illustrated inTable 16.

                  TABLE 16                                                        ______________________________________                                        EFFECT OF NEGLECTING THE BACKGROUND                                           CORRECTION ON THE ACCURACY OF                                                 CHEMOMETRIC PREDICTIONS                                                                       Bias     SEP                                                  Constituent     (wt %)   (wt %)                                               ______________________________________                                        n-Heptane       -0.47    0.63                                                 i-Octane        0.16     0.23                                                 Toluene         0.37     0.41                                                 p-Xylene        -0.21    0.24                                                 n-Decane        0.14     0.41                                                 n-Alkanes       -0.33    0.38                                                 ______________________________________                                    

Table 16 illustrates that ignoring the background correction in aspectrophotometer having an attenuator and a mode scrambler providedreasonable accuracy. The biases in the prediction of concentrationaveraged about 0.3 wt %.

EXAMPLE 8

The effect of drift in the background spectrum, which can be caused byinconsistency or non-repeatability in the high-efficiency fiber opticswitches, on the accuracy of chemometric predictions was determined forthe spectrophotometric process of the present invention. Light launchedfrom a fiber optic cable onto a flat surface generally images as afairly homogeneous spot. Light launched through a switch generallyimages as a small, intense central spot surrounded by a more diffusespot. Upon cycling, the spot can move slightly. When the switch launcheddirectly into the spectrograph, this particular movement resulted in anapparent shift in the wavelength axis and errors in chemometricpredictions. This effect was reduced by inserting a mode scramblerbetween the switch and the spectrograph. The mode scrambler made imagesfrom the switch more uniform both in spot size and in the radialdistribution of light in the spot.

The mode scrambler used in this Example was made by wrapping the fiberoptic cable around two half inch diameter spots in a figure eightpattern, using four complete repetitions of the figure eight pattern.The drift, for purposes of this Example, is given by the switchingnoise, which is defined as the maximum difference in thefirst-derivative spectra before and after 100,000 switching cycles at 20Hz. For the control case, the sample and reference spectra were obtainedusing the same channel without cycling the switch. The typicalphotometric range in the first-derivative spectrum of a sample was 0.02dABS, about 400 times greater than the noise given by the control. Theresults of mode scrambling, as described above are illustrated in Table17.

                  TABLE 17                                                        ______________________________________                                        THE EFFECT OF MODE SCRAMBLING ON THE                                          SWITCHING NOISE, WHICH IS RELATED TO DRIFT                                    IN THE BACKGROUND SPECTRUM                                                                        Peak-to-Peak                                                                  Switching Noise                                                               (dABS × 10.sup.4)                                   ______________________________________                                        Control, no switching 0.5                                                     No mode scrambler     5                                                       Mode scrambler        0.9                                                     Two mode scrambler in series                                                                        1                                                       Mode scrambler in series with                                                                       0.7                                                     700 ft of fiber optic cable                                                   Mode scrambler with cladding mode                                                                   2                                                       filter                                                                        ______________________________________                                    

Table 17 illustrates that in the absence of a mode scrambler, the noiselevel after 100,000 switching cycles was 10 times larger than thecontrol or the inherent noise level of the instrument with thehigh-efficiency fiber optic switches held stationary. When the modescrambler was used, the noise was only twice that of the control.Attempts to decrease switching noise by placing two mode scramblers inseries and placing a mode scrambler in series with a long run of fiberoptic cable provided minimal benefit. Similarly, a mode scrambler thatstripped out cladding modes corresponding to light which propagates inthe cladding rather than the core of the fiber optics, resulted in aslightly smaller, sharper spot, but did not reduce the switching noisefurther.

The noise from cycling the high-efficiency fiber optic switches wascompared with the noise caused by bending the fiber optic cable and bymisaligning the fibers in the fiber optic switch. Either of theseeffects can cause switching noise. The noise was further measured forswitches made from fluorocarbon clad fibers (FCS) and glass clad fibers(GCS), and with an without a mode scrambler. For the control, theswitches were cycled 100,000 times. For the other tests, the fiber opticleading to the sample cell was bent on a three-inch radius, and thefiber was misaligned by shimming the stop with a thin plastic sheet,which reduced the overall light transmission through the switch by10-20%. The mode scramblers used in this Example were made from FCSfiber wrapped around two half-inch (1.3 centimeters) diameter spools ina figure-eight pattern, using four complete repetitions of thefigure-eight pattern. The results of the comparison are illustrated inTable 18.

                  TABLE 18                                                        ______________________________________                                        EFFECTS OF FIBER TYPE AND MODE SCRAMBLING                                     ON THE NOISE CAUSED BY CYCLING THE FIBER                                      SWITCHES, BENDING THE FIBER OPTICS AND                                        MISALIGNING FIBERS IN THE SWITCH                                              Noise in dABS × 10.sup.4                                                FCS Fiber                                                                     without       FCS Fiber GCS Fiber  GCS Fiber                                  Mode          with Mode without Mode                                                                             with Mode                                  Scrambler     Scrambler Scrambler  Scrambler                                  ______________________________________                                        100,000 3         0.8       2.5      1.7                                      switching                                                                     cycles                                                                        Bending 2.5       1.5       4        1.5                                      the fiber                                                                     Misaligning                                                                           3         1.2       8        5                                        the fiber                                                                     ______________________________________                                    

Table 18 illustrates that it is unclear as to whether the switchingnoise was caused by bending or misalignment of the fiber optics.However, the mode scrambler reduced noise from all three sources and issuitable and beneficial for use with spectrophotometric processesutilizing fiber optics. The switches made with FCS fiber optic cablewere less noisy than those made with GCS fiber optic cable. It isbelieved that this is related to the fact that the light output changedmuch more gradually with the angle of propagation in the FCS fiber. TheFCS fiber optic cable is generally preferred over the GCS fiber opticcable for use in the high-efficiency fiber optic switches because it isless noisy. Adversely, however, the FCS fiber optic cable also absorbedmore light than the GCS fiber optic cable, so the GCS fiber is preferredin long runs of fiber optic cable. When a mode scrambler was used,cycling the switches caused little error in the chemometric predictions.

The effect of correcting for the change in the background spectrum wasmeasured. Chemometric models were tested before and after about2,000,000 cycles, which gave a switching noise of 0.0002 dABS. Thechange in the background spectrum was also measured and the switchingnoise was removed by correcting for the drift in the backgroundspectrum. The results of correcting for the background spectrum areillustrated in Table 19.

                                      TABLE 19                                    __________________________________________________________________________    EFFECT OF CYCLING THE FIBER OPTIC SWITCHES ON THE ACCURACY OF                 CHEMOMETRIC PREDICTIONS                                                       __________________________________________________________________________            CASE 1               CASE 2                                                         AFTER AFTER          AFTER AFTER                                        INITIAL                                                                             CYCLING                                                                             CORRECTION                                                                             INITIAL                                                                             CYCLING                                                                             CORRECTION                           __________________________________________________________________________    BIAS (wt %)                                                                   n-Heptane                                                                             -0.04 -0.75 --       -0.15 1.60  1.49                                 i-Octane                                                                              -0.03 0.12  --       -0.04 -0.17 -0.18                                Toluene -0.04 -0.18 --       0.02  0.25  0.21                                 p-Xylene                                                                              0.14  0.13  --       0.02  -0.29 -1.32                                n-Decane                                                                              -0.02 0.68  --       0.16  -1.38 -1.19                                n-Alkanes                                                                             -0.07 -0.06 --       0.01  0.22  0.30                                 SEP (wt %):                                                                   n-Heptane                                                                             1.14  0.89  --       0.51  1.86  1.73                                 i-Octane                                                                              0.17  0.18  --       0.14  0.25  0.26                                 Toluene 0.18  0.22  --       0.08  0.29  0.26                                 p-Xylene                                                                              0.21  0.15  --       0.05  0.35  0.38                                 n-Decane                                                                              0.94  0.80  --       0.38  1.59  1.39                                 n-Alkanes                                                                             0.22  0.10  --       0.13  0.29  0.37                                 __________________________________________________________________________            CASE 3               CASE 4                                                         AFTER  AFTER         AFTER AFTER                                        INITIAL                                                                             CYCLING                                                                             CORRECTION                                                                             INITIAL                                                                             CYCLING                                                                             CORRECTION                           __________________________________________________________________________    BIAS (wt %)                                                                   n-Heptane                                                                             -0.32 1.22  1.70     0.07  -0.26 -0.21                                i-Octane                                                                              0.06  -0.37 -0.30    -0.04 0.09  0.05                                 Toluene -0.04 0.30  0.26     0.03  -0.31 -0.19                                p-Xylene                                                                              -0.06 -0.09 -0.14    -0.01 0.13  -0.02                                n-Decane                                                                              0.35  -1.05 -1.52    -0.12 0.34  0.35                                 n-Alkanes                                                                             0.03  0.17  0.18     -0.05 0.07  0.14                                 SEP (wt %):                                                                   n-Heptane                                                                             0.50  1.43  1.96     0.55  0.56  0.54                                 i-Octane                                                                              0.12  0.44  0.37     0.46  0.55  0.55                                 Toluene 0.04  0.34  0.29     0.42  0.55  0.49                                 p-Xylene                                                                              0.09  0.19  0.22     0.22  0.28  0.24                                 n-Decane                                                                              0.52  1.26  1.77     0.67  0.81  0.82                                 n-Alkanes                                                                             0.04  0.20  0.21     0.31  0.42  0.44                                 __________________________________________________________________________

Table 19 illustrates that the switching noise, which was removed bycorrecting for the drift in the background spectrum, caused relativelylittle bias in the predictions. Most of the errors could be attributedto drift in the wavelength axis caused by thermally induced movementsbetween optical components of the spectrograph, rather than cycling ofthe high-efficiency fiber optic switches. For the last replicate inTable 19, where an effort was made to minimize the wavelength drift, theSEPs for each constituent differed by about 0.1 wt % before and aftercycling.

EXAMPLE 9

The effect of increasing and decreasing sample temperature, on theaccuracy of chemometric predictions was determined for thespectrophotometric process of the present invention. The temperaturedependence of absorbance was measured for toluene, iso-octane,cyclohexane, carbon tetrachloride, and reformate. The optics of thesample cell were mounted in a housing designed for an in-line probe. Theprobe was placed in a test stand where the sample temperature could beincreased at constant pressure. The change in absorbance, averaged overall wavelengths, was calculated by linear regression at eachtemperature. For each sample, the apparent thermal expansion wasdetermined from the change in absorbance.

For carbon tetrachloride, the control, the maximum difference in thespectra between 75° F. (23.9° C.) and 186° F. (85.6° C.) was 0.0002dABS, which was substantially caused by switching noise. The apparentcoefficients of expansion for iso-octane, toluene, and reformate(0.05%/°F. or 0.09%/°C., 0.06%/°F. or 0.11%/°C. and 0.06%/°F. or0.11%/°C.) were close to the literature values of 0.06%/°F. (0.11%/°C.).The value for cyclohexane was much less than expected (0.04%/°F. or0.07%/°C. as compared to 0.07%/°F. or 0.13%/°C.), and this wasattributed to cyclohexane undergoing conformational changes in thistemperature range which may have caused the unexpected result.

The composition of mixtures of benzene and iso-octane were predictedover a temperature range of from about 75° F. (23.9° C.) to about 187°F. (86.1° C.) and predictions were made with and without correcting forthermal expansion. The chemometric model was built at 75° F. (23.9° C.)and a coefficient of expansion of 0.06%/°F. (0.11%/°C.) was used. TheSEPs were calculated for each comparison test. The results of thesecomparison tests are illustrated in Table 20.

                                      TABLE 20                                    __________________________________________________________________________    HOW TEMPERATURE AFFECTS THE PREDICTION OF WT % BENZENE IN ISO-OCTANE                              ERRORS OF PREDICTION                                                                         ERRORS OF PREDICTION                       MODEL WITH 2 LATENT (Wt %) FOR 60.5%                                                                             (Wt %) FOR 89.7%                           VECTORS             BENZENE SOLN.  BENZENE SOLN.                                        MODEL ERROR                                                         Wt % BENZENE                                                                            (Wt %)    °F.                                                                       RAW CORRECTED                                                                             °F.                                                                       RAW CORRECTED                           __________________________________________________________________________     0.0      1.7        76                                                                              1.0 1.0      75                                                                               0.3                                                                              0.3                                 10.1      -1.3       84                                                                              1.0 1.0      84                                                                              -0.8                                                                              -0.6                                21.8      -0.8       95                                                                              0.9 1.0      95                                                                              -0.8                                                                              -0.4                                30.7      -0.6      105                                                                              0.8 0.8     105                                                                              -1.0                                                                              -0.4                                41.2      0.7       118                                                                              0.7 0.8     116                                                                              -0.9                                                                              -0.1                                51.3      -0.4      126                                                                              0.7 0.8     124                                                                              -0.8                                                                              0.2                                 60.5      0.5       137                                                                              0.6 0.7     136                                                                              -0.8                                                                              0.3                                 69.7      0.6       148                                                                              0.6 0.6     144                                                                              -0.9                                                                              0.4                                 80.9      0.1       158                                                                              0.5 0.6     155                                                                              -0.8                                                                              0.5                                 89.7      0.9       164                                                                              0.5 0.5     166                                                                              -0.9                                                                              0.6                                 100.00    1.0       174                                                                              0.4 0.5     177                                                                              -1.0                                                                              0.7                                 100.00    -1.9      186                                                                              0.3 0.5     187                                                                              -1.2                                                                              0.7                                 Bias      --           0.7 0.7        -0.8                                                                              0.2                                 Std. Dev. 1.1          0.2 0.2         0.4                                                                              0.4                                 __________________________________________________________________________

Table 20 illustrates that the corrected and uncorrected SEPs were withinthe modeling error (SEE) of 1.1 wt %. However, the corrected predictionshad less bias for the high benzene concentration. Although the effect ofsample temperature was small, the error in the predictions decreasedwhen a correction was made for thermal expansion. Changing the sampletemperature introduces artifacts or structures into the spectra that aredifficult to compensate for. A chemometric model can be built over abroad range of temperature and composition, but a large number ofsamples would be required. Controlling the sample temperature within arange of 40° F. (22.2° C.) and more preferably 30° F. (16.7° C.) forcompounds without hydrogen bonding would limit the appearance ofartifacts, and correcting for thermal expansion would provide firstorder compensation for the effects of temperature.

EXAMPLE 10

The effects of sample flowrate on the accuracy of chemometricpredictions were determined by comparing the spectra of a reformatestream at flowrates of 0 ccm and 500 ccm. At 500 ccm, the transmissionof light through the sample was attenuated by 80% to 90%. However thefirst derivative spectra of absorbance appeared substantially similarfor the two flowrates. Flow through the sample probe was not fullydeveloped into an equilibrium velocity distribution in the opening ofthe sample cell, but the maximum velocity at 500 ccm was 0.1 to about1.0 ft/sec (0.3 meters/sec). The attenuation of the light transmissionthrough the sample was caused by scattering from either gas bubbles ordensity fluctuations in the liquid. Flowrate is unlikely tosubstantially effect chemometric prediction accuracy if the process issampled through a slipstream and the maximum velocity in the sample cellis maintained below 1 ft/sec (0.3 meters/sec) and preferably below 0.1ft/sec (0.03 meters/sec). However, in order to transmit enough lightthrough the sample, it may be necessary to reduce the liquid velocitythrough an in-line probe to similar levels.

That which is claimed is:
 1. A process for obtaining spectralinformation and quantifying the physical properties of a samplecomprising:launching polychromatic light having a wavelength rangingfrom about 100 nanometers to about 2500 nanometers alternately throughat least one sample channel and at least one reference channel, througha single fiber optic strand and at least one high-efficiency fiber opticswitch having a coupling efficiency of not less than 50 percent; passingsaid polychromatic light along said sample channel to said sample cellcontaining said sample wherein said polychromatic light is passedthrough said sample and sample spectral information produced; routingsaid polychromatic light along said reference channel and producingreference spectral information; reproducibly and uniformly imaging saidsample and reference spectral information from said sample and referencechannels by passing said spectral information through a mode scrambler;processing said uniformly imaged sample and reference spectralinformation from said mode scrambler in a spectrograph wherein saiduniformly imaged spectral information is separated into componentwavelengths and the light intensity at each wavelength determined andrecorded utilizing a photodiode array detector; and utilizing saidseparated and recorded uniformly imaged sample and reference spectralinformation to predict said physical properties of said sample.
 2. Theprocess of claim 1 wherein said polychromatic light is in the wavelengthranges of from about 800 nanometers to about 2500 nanometers.
 3. Theprocess of claim 1 wherein said polychromatic light is generated from atungsten-halogen lamp.
 4. The process of claim 1 wherein saidpolychromatic light is alternately launched through said sample channeland said reference channel using a high-efficiency fiber optic switchcycling at a frequency ranging from about 10 cycles per second to about0.01 cycles per second.
 5. The process of claim 4 wherein said utilizingstep comprises generation of filtered absorbance spectra and said modescrambler reduces noise irreproducibilities in said filtered absorbancespectra, produced from said high-efficiency fiber optic switch, by afactor of at least 3 in the filtered absorbance spectra derived fromsaid separated and recorded uniformly imaged sample and referencespectral information, over said process without a mode scrambler.
 6. Theprocess of claim 1 wherein said reproducibly and uniformly imaging stepcomprises directing sample and reference spectral information along fromabout 60 meters to about 600 meters of fiber optic cable.
 7. The processof claim 1 wherein wavelength drift in said wavelength discriminationdevice is maintained at less than 0.30 nanometers.
 8. A process forobtaining near infrared spectral information and quantifying thephysical properties of a sample comprising:launching polychromatic lighthaving a wavelength ranging from about 800 nanometers to about 2500nanometers alternately through a single fiber optic strand and at leastone sample channel and at least one reference channel, through at leasttwo high-efficiency mechanical motion mechanism fiber optic switches;directing said sample to a sample cell; passing said polychromatic lightalong said sample channel to said sample cell wherein said polychromaticlight is passed through said sample and sample spectral informationproduced; attenuating said polychromatic light along said referencechannel and producing attenuated reference spectral information forbalancing polychromatic light transmission between said reference andsample channels; reproducibly and uniformly imaging said sample spectralinformation from said sample channel and said attenuated referencespectral information alternately by passing said spectral informationthrough a mode scrambler; processing said uniformly imaged sample andattenuated reference spectral information from said mode scrambler in aspectrograph wherein said uniformly imaged spectral information isdiffracted into component wavelengths and the light intensity at eachwavelength determined and recorded utilizing a photodiode arraydetector; and utilizing a chemometric model and said diffracted andrecorded uniformly imaged spectral information to predict said physicalproperties of said sample.
 9. The process of claim 8 wherein saidpolychromatic light is in the wavelength range of from about 800nanometers to about 1100 nanometers.
 10. The process of claim 8 whereinsaid sample is crude petroleum and said polychromatic light wavelengthspans at least one range selected from the group consisting of fromabout 1300 nanometers to about 1500 nanometers and from about 1600nanometers to about 1800 nanometers.
 11. The process of claim 8 whereinsaid polychromatic light is alternately launched through said samplechannel and said reference channel from a high-efficiency fiber opticswitch cycling at a frequency of from about 10 cycles per second toabout 0.01 cycles per second.
 12. The process of claim 8 wherein saidutilizing step comprises generation of filtered absorbance spectra andsaid mode scrambler reduces noise irreproducibilities in said filteredabsorbance spectra, produced from said high-efficiency fiber opticswitches, by a factor of 3 in the filtered absorbance spectra derivedfrom said diffracted and recorded uniformly imaged sample and referencespectral information, over said process without a mode scrambler. 13.The process of claim 8 wherein said photodiode array detector operatesin a range not exceeding 85% of full scale light intensity required tosaturate said detector.
 14. The process of claim 8 wherein thephotometric response of said spectrograph is substantially linear tobetter than 0.9% of full scale light intensity.
 15. The process of claim8 wherein said spectrograph temperature is maintained within 5.6° C. ofa constant operating temperature target.
 16. The process of claim 8wherein wavelength drift in said spectrograph is maintained at leastthan 0.30 nanometers.
 17. The process of claim 8 wherein wavelengthdrift in said spectrograph is maintained at less than 0.03 nanometers.18. The process of claim 8 wherein the resolution of said spectrographis better than 4 nanometers.
 19. A process for obtaining near infraredspectral information and quantifying the physical properties of a samplecomprising:launching polychromatic light having a wavelength rangingfrom about 800 nanometers to about 1100 nanometers alternately throughat least one sample channel and one reference channel, through a singlefiber optic strand and at least two high-efficiency latching typemechanical motion fiber optic switches; directing said sample to asample cell; passing said polychromatic light along said sample channelto said sample cell wherein said polychromatic light is passed throughsaid sample at least twice utilizing a light reflection device, whereinsaid polychromatic light contacting said light reflection device isreflected at least twice, and sample spectral information produced;attenuating said polychromatic light along said reference channel andproducing attenuated reference spectral information for balancingpolychromatic light transmission between said reference and samplechannels; reproducibly and uniformly imaging said sample spectralinformation from said sample channel and said attenuated referencespectral information alternately by passing said spectral informationthrough a mode scrambler; processing said uniformly imaged sample andattenuated reference spectral information from said mode scrambler in aspectrograph wherein said uniformly imaged spectral information isdiffracted into component wavelengths and the light intensity at eachwavelength determined and recorded utilizing a photodiode arraydetector; and utilizing a chemometric model and said diffracted andrecorded uniformly imaged spectral information to predict said physicalproperties of said sample.
 20. The process of claim 19 wherein saidpolychromatic light wavelength ranges from about 850 nanometers to about1000 nanometers and said sample is a hydrocarbon liquid which istransparent or partially transparent over said wavelength range.
 21. Theprocess of claim 19 wherein said polychromatic light is alternatelylaunched through said sample channel and said reference channel at ahigh-efficiency fiber optic switch cycling frequency of from about 1cycle per second to about 0.1 cycles per second.
 22. The process ofclaim 19 wherein said sample is conditioned and maintained within 11.1°C. of a constant operating temperature target.
 23. The process of claim19 wherein said passing step for producing sample spectral informationcomprises passing said polychromatic light through said sample twiceutilizing a light reflection device wherein said said polychromaticlight contacting said light reflection device is reflected three times.24. The process of claim 19 wherein said sample passes through saidsample cell at a velocity of less than about 0.3 meters.
 25. The processof claim 19 wherein wavelength drift in said spectrograph is maintainedat less than 0.03 nanometers.
 26. The process of claim 19 wherein theresolution of said spectrograph is better than 2 nanometers.